Having the water supply come up through the floor behind the bowl does limit your choices.
If cost is a factor, then any 12" bowl would work, even it it is a 14" rough.
"if" the water supply isn't in the way.
You should be able to install what you have though. Tanks can come down tight, or they can have some play. As long as they remain water tight it should be okay. If you are worried about the movement, you options are to shim, or the brace to the wall.
We don't, but then the toilets we've been installing have been fairly solid.
Don't expect to be able to use any toilet as a lazy boy recliner though. With enough weight pushing against a porcelain tank and two bolts, anything can happen.