What you need to tell the electrition is that you want a star ground system. That is where there is a dedicated line and ground wire for each outlet all the way back to the panel.
It sounds like to don't need to worry about the same phase becuse your panel is full anyway. When they hook up the sub panel they will put it on one of the 2 phases (each side of the 220). If you can get them to put the sub panel on and secondarys off of the power meter it will be even more isolated. If not have him pick the side of the phase (of the 220 that has the least amount of noisy stuff on it, like refirges,moters, air conditioners exc... good luck...