How does Red Guard / waterproof relate to floating for tile?
You use Redguard after the floor is prepped. You can do minor floating as in a thin layer of thinset to fill in an area that may have been overlooked while floating the bath floor the first time.

Being its a condo and the floor is concrete, the floor is either poured in place or precast panels set in place. That in mind, I can't see the floors being much out of whack that would make the floors difficult to get right. Worst case with a precast floor, you may have a joint where two precast slabs meet, that would be the most problematic. If there is two distinct levels then there is not enough grinding that will fix it, better to use a self leveling underlayment with their approved primer, the add the Redguard. You can still touch it up a bit with thinset if you feel it is needed. 

The larger tile will take a LARGE notched trowel (1/2"X1/2")to set the floor. This itself will automatically build in some give and take. A level floor is good, but you can work with a floor that is all on the same plane. In other words, flat, no high spots, maybe just a little out of level, not much though.