Any difference for a pressure-assisted toilet?
It looks like there's no difference in plumbing requirements from this installation guide.
This regular toilet installation guide lists the drain and vent requirements:

it requires its own 2&quot; minimum vent and a drain of at least 3&quot; in diameter

As far as retrofitting a pressure assist vessel into an old gravity flush toilet, I wouldn't try it. Pressure assist toilets are designed differently than gravity flush ones. Flushmate's website warns:

The FLUSHMATE® Pressure-Assist system cannot be retrofitted into a standard, gravity-fed toilet.

There might be a retrofit kit out there somewhere, but I'd be wary about how well it would work.