I've currently got my 5 easterns and 3 checkereds living together, and I've seen no problems with the biggest eastern at around 20 inches and the biggest checkered a little over 2 feet. The other two checkereds are around 8 inches. :P
Just make sure if you're feeding them communally that everyone has something in their mouth at the same time, and if they're taking scented items, worms or fish, make sure to get all the smell off them via rinsing.