This is less "of a how do I?" than a "should I?" question.  I am doing a wedding cake in about a month, and the bride originally wanted 2 tiers all fondant covered, but now has decided she wants a completely different design.  The design is a 4 tier, the top tier is quilted, the next tier is swirl roses, then smooth with a ribbon, then the last tier is swirl roses again.  It's totally gorgeous, but  my question is this.  I have no worries at all about the tiers with the roses as I have done them many times, but I worry about doing the other 2 tiers in buttercream.  It's so hard to get buttercream super super smooth, and then putting a ribbon around it, and quilting it...  I fear I am going to make a mess.  What do you all think about/have you ever seen a cake that has some tiers buttercream and some tiers fondant? It will be ivory so I don't have to worry about the buttercream not being pure white.  I would appreciate any advice and opinions!

AI personally wouldn't mix fondant tiers with bc tiers. I did a cake last year that was 6 tiers all bc, quilted, with pearl dot centers on each cross of the quilting. I also had the same question- how will the quilting look on bc?? But it turned out lovely. You just need to make sure to not quilt until your bc has gotten a crust, otherwise it will just gum up everywhere. Full disclosure, I use a ps-based all butter buttercreme. I don't know how it would work with a shortening creme.

AOh, and you can put a wax paper "backing" on your ribbon so that the oil doesn't bleed through your ribbon (plus keep the nonfoodsafe ribbon off your buttercreme.)

When I do this I let it crust, smooth it with the Viva method, then put it in the freezer for about 2 minutes. I use a quilting impression mat dusted with cornstarch. HTH

I use an italian meringue buttercream, and it doesn't crust.  It does get pretty stiff when it's refrigerated, though.  I am a little stressed about this!  The original design was quite simple, and this is quite a bit more difficult.  It's only my second wedding cake!  Perhaps it would be best to use a different buttercream for the quilted layer?!?  Oh goodness, lol.

Im of a different opinion, there is going to be so much texture difference between the tiers with smooth icing and the roses one, that I think fondant will look good!

I can never get butter cream smooth so personally I will go for fondant with the rose swirls on the other tiers

btw I think ps-based is powdered sugar based :)

I have seen quite a few rosette cakes with a fondant tier, I think they look great, I actually prefer them to the all rosette styles. Quilting on butter cream is tricky, especially if you don't work with a crusting one.

A super quick google search came up with these, just to show that it doesn't look strange or 'clashy'.

https://lovellabridal.com/blog/wp-content/uploads/2011/08/2_couture_wedding_cake_nancykays_confections_grace_ormonde_wedding_style.jpg

http://crabbycakecreations.files.wordpress.com/2012/04/rosesandpearls.jpg

AOf course it means powdered sugar, omg how did I not figure that out?!? Lol. It was late!!! Thank you for your advice everyone!

The cake that I have as my avatar is done with BC rosettes and a fondant tier. Perhaps it will help you decide if you like the look or not. 

I would do your smooth tiers in fondant.  Obviously you can't quilt on IMBC and if you don't have any experience with crusting buttercreams, I would just use fondant.

Lots of people mix fondant and buttercream tiers.  You usually see this when the buttercream tier is textured (ruffles, rosettes, etc.).  Here are a few examples from DonnaOK:

http://cakecentral.com/g/i/3006628/a/3349572/one-of-my-brides-getting-married-this-month-wanted-a-special-cake-for-her-birthday-she-told-me-she-wanted-one-tier-round-one-tier-square-and-she-loved-pink-i-hand-piped-the-bottom-tier-in-buttercream-ruffles-i-covered-the-top-tier-in-pink/

http://cakecentral.com/g/i/2167802/a/2168802/i-loved-creating-this-cake-i-covered-the-cake-in-white-fondant-and-then-i-hand-stenciled-the-purple-chandeliers-on-i-covered-the-middle-tier-in-rosettes-and-then-added-glitter-dust-to-the-middle-tier-i-hand-piped-a-zig-zag-top-border-i-then/

http://cakecentral.com/g/i/2388062/a/2389062/theres-something-to-be-said-about-an-all-white-wedding-cake-my-bride-shannon-was-so-elegant-and-she-wanted-to-incorporate-the-rosette-pattern-with-the-quilted-pattern-i-did-the-top-and-bottom-tier-in-white-buttercream-rosettes-then-the-middle/