I have a 7.5 HP three phase rotary convertor, wired to a three phase box, with breakers for the 509G, a Bridgeport, and a surface grinder.
I have another rotary wired to a 240/480 step up transformer, to supply 480-3ph to a Hardinge lathe, I got this rotary on the cheap, so I used it rather than wire the transformer to the original rotary.
The big Grizzly has a pretty good size motor on it, since I run it the slower motor position 100% of the time, the 7.5 is plenty. Uses more current on high BTW.
You can't go wrong, if you get a minimum 7.5 HP unit, and have breakers for each piece of equipment. Also, please, remember to ground everything - this is very important for safety, this stuff can kill you if it goes to ground thru you. Get a real electrician if you don't understand wiring completely.