If size was going to be an issue, then that should have been discussed before the purchase.
That water has already passed over the dam. And if you gave him an okay to do the work, it's done.
All ADA toilets will be 1.5" longer.
And then not all are the same. Checking the spec sheets will let you know beforehand which ones are more compact.
A round bowl will be from 26.5" to 28" in depth
An elongated bowl (ADA) can be 28" to 30"
All of these will have some offset from the wall on a standard 12" rough.
If you need compact and tall, then the choices narrow
TOTO does make a CST423 bowl, Promenade, that is tall and round.\
Some of the dual flush bowls have a button on top. I've haven't had any problems with those.