Would a 6/9 serve 30 people?  It's a party cake, not a wedding cake.  Or is there another combination of sizes that would work better?  Thanks!

I have done a 3 layer 8" and 3 layer 6" and it has more than enough fed 30+ people.

A6/9 is more cake than you need.

I think a better combination would be a 8" plus 10" (double layers). Don't skimp, and it is better to have enough servings of a delicious cake for seconds.  If any leftovers, guests will be happy to take home, or cakes are easy to freeze for later. If decorated, freeze first, then wrap in foil. To defrost, loosen foil but leave covered. 

AYou wont make any $ with that advice. Assuming this is a paid ordet. And the customer is willing to pay for WAY more cake than they need. Using the W party guide 6/9 is 30 servings. And around here we always say to *price* by the W wedding chart servings. If the customer wants bigger servings or leftovers they buy more cake. Of course if this is a gift, do whatever you think is best.

And an eight inch for a top tier just generally looks odd, unless you have a giant topper to balance out that huge expanse.

I use this when I do every cake order for size reference!  Amazing Cake Lady who now just sells tools for decorating but very helpful in resources!   Print this and I hope it helps with each order you do from here out!!  Can't wait to see pictures!

http://www.earlenescakes.com/ckserchart.htm

AI do not like the 8" as a top tier...to me it looks large and strange. Have you don't a 4" cake before? Or maybe a 5/7.. I do 6/8 all the time and that's about 25 maybe? Don't have my chart here