Oops, commented on the pic instead of your thread.

Did you use dams with your fillings?

What did you use for supports?

It looks as if your filling layers might be fairly thick. Keep the filling layers thin.  to get more filling in your cake, consider making a 3 or 4 layer cake with thin layers of filling. Good tangy homemade lemon curd is so flavorful a little goes a long way.   For lemon curd, i cook it an extra 15 minutes at 190 degrees (just below boiling) to make it really thick.  Then also make a very stiff buttercream ( i usually use SMBC for the outside of my cakes but I make a stiff ABC with extra powdered sugar) to use as a dam around the outside edges of your fillings if they are soft.  Pipe a circle of ABC around the outside edge of each layer,, but about 1/2 "  in from the edge.  That way if there is some squishing outward, it is less likely to make it to the edge and show.  You can fill in the gaps later with your outer frosting.  I then put the layer with the dam in the freezer for 10 mins to make the dam firm, then spread the filling inside, but no deeper than the dam.  Stack the layers.  Also a very nice trick from Leah_s is to place a tile on top of the stacked cakes to let them settle and get the bulges out before the final frosting.See the link below.   Also of course, a good support system is essential to take the weight off of the individual tiers. 

http://cakecentral.com/t/633571/my-newest-trick

Wow- thanks so much! What great ideas! I am baking one trial cake now and will try this out to prevent bulging!

Any tips to arrange the roses? FYI, I want to ultimately go for this look:

http://cakecentral.com/g/i/3077773/a/3389447/need-help-making-this-cake-trying-to-figure-out-1-pan-sizes-to-use-trying-to-avoid-16-pan-looking-to-use-6-8-9-10-12-14-or-4-6-8-10-12-14-or-if-i-must-use-16-then-6-8-10-12-14-16-not-worried-about-of-servings-b-c-there-will-be-plenty-of/

Also forgot to mention how I stacked it:

Between the bottom and middle tier, I used (2) wilton separator plates basically directly on top of each other directly on top of the bottom cake, so that the distance between the two cakes is just the height of the spikes if that makes sense (spikes pointing at each other) - I would have put some pillars between but I needed the space between the two cakes very shallow so that the roses would hide them, so that is how I ended up with this. To support the plates and upper two layers, I did put (4) wilton plastic pegs that snapped into the bottom cake plate.

Between the middle and the top, I put the top cake on a 6" separator plate, and put (4) wilton hidden pillars (plastic tube like, about 1" in diameter) that went down into the middle cake. I didn't have an extra separator plate, and the top tier was light enough that I figured it would not need support beyond this.

Is there a better way to support/stack this? What would everyone recommend???

Keep in mind for the real one, I plan to go with (6) tiers ... Please help! Thanks in advance!!!

(fyi plastic pegs: http://www.wilton.com/store/site/product.cfm?id=3E311B76-475A-BAC0-5EF476FB742AB190&killnav=1

hidden pillars: http://www.wilton.com/store/site/product.cfm?id=3E3119F0-475A-BAC0-5772682F766C019C&killnav=1

separator plates:http://www.wilton.com/store/site/product.cfm?id=3E30FE88-475A-BAC0-5D58F82BF466B5B5&killnav=1

AYou have to buy roses that will open, to get open roses. That kind stays in the bud shape until they wilt. And you should never jab flowers into your cake. The should be put into straws, or wrapped in foil to keep the muck out of the cake.

Are you aware it is also tipping?

for the flowers to be like the flowers in the photo you like, you would need to get gumpaste roses and filler.  If you aren't up to making all those roses, then you can buy them from places like cal java online (or others). If you want to use fresh flowers, like Anna said, you need to have full roses.  The problem with big fresh flowers is that once they blow open, they are just about ready to shed pedals (unlike fresh garden roses that bloom open and are still vibrant because they are still alive).  If you want a fully open fresh rose look in a real flower, try using tea roses that are opened, they seem to be more sturdy when opened.  Also, don't stick fresh flowers directly into a cake. To get that horizontal profile of the flower instead of the vertical placement in your test cake, you need to support the base and cut stem of the flower in a blob of buttercream and then rest the flower on its side so that you can see the full head of the bloom. You will also probably need 4X the number of flowers that you have in your test cake. It's much better to have too many and then place the extras in small vases on the cake table, then to have to snag flowers from the centerpieces to fill in your spaces. 

oh...and yes on the SPS.  A much easier system that will really help out your cakes overall (especially until you get your recipes and constuction of tiers a bit more perfected without bulges and sliding) 

Thank you so much for your valuable advice!

Do you know where I can find "fresh" roses that will open like this? Is there a difference between long-stem and short-stem?

Since I need so many, is there a way to buy flowers in bulk? I figure I need approx. 5 dozen roses ...

Thanks again!

You can look for a wholesale florist that sells to the public. In my area the local one is called  both Roses and More and The Secret Garden. I got my wedding roses there, 16 dozen, and it was about $120. They will help you pick a rose, and tell you the best time to order them, to insure they are open in time for the wedding. Other than that, there is Sam's Club. 

Gum paste is great option. And HIGH QUALITY silk flowers, this late in the game.... Hobby Lobby or Michael's. 

My favorite florist provided 4" blooms for a wedding cake once, they were so gorgeous, I was in awe. $6 a pop, loose. Call around.

Thanks so much for all of your great advice- this is such an amazing network of bakers and cake enthusiasts!

Final questions: This is the first time I have heard of the SPS system-- is there any way I can buy or rent this system in time? The wedding is in (6) days (next Saturday) in Kansas City, Missouri.

Where can cake support systems/tiers be rented from? Do you just call around bakeries to see who will rent to you? I have never heard of renting before, but this might be a good option for me since I don't make stacked/tiered cakes very often ...

It sounds like everyone is discouraging me from using Wilton systems-- I'm just worried I won't be able to get an SPS system in (6) days- does anyone have any luck using Wilton with a large (6) tier cake? If so, any advice/tricks? I will try to find the other system, but have the Wilton parts as a back up if I can't find in time ...

Lastly, if I buy roses from Sam's Club, do I have to order them in advance online or are they right there in the store (in bulk, 5+ dozens of white roses)?

Thanks so much, everyone! I really appreciate your much needed support!!!!!

ACall around to local cake supply stores and ask if they carry it. You'll have to buy it, it's not expensive, pretty equivalent to the wilton..just waay better. I'm in your general area, the Sweet Supply Shoppe in Overland Park carries it - if you can't find another store closer. They're not open on Sundays/Mondays so check around your area first. But start now so you're not in a last minute panic!

Very informative!

Maybe you can call Global Sugar Art & see if they can ship overnight or something. The phone number is on their website:

https://www.globalsugarart.com/index.php

Worth a try!

Thanks so much sixinarow!!!!! I will call them first thing on Tuesday when they open!