Slap a fermometer on each bucket? That way you'll know the temp of each one.
My concern is that 70 is high to turn the AC on, the beer could be much hotter. The heat of fermentation will take a long time to warm up a room that size, so the beer could be at 75 while the room is at 65. I would probably set the heater to click on at 62 and the AC at 65. Play with it though, and you'll figure out a sweet spot.
Allow space between the new buckets to let heat dissipate, and you can cluster the older ones together since they won't be as active.