I would definitly get the large for tank for those guys, pearsei (as stated before) are massive fish. As far as overstocking goes its more then possible to overstock a c/a tank if done right... but it all depends on what kind of tank you are setting up. I would never mix spieces together (vieja w/ vieja) unless they were a breeding pair. In wich case I would give them there own tank or have a good dither or two in it. But if you do like a parachromis, a vieja, a nandopsis, etc. Useually things are good. But try to avoid the super aggresive ones as they are harder to do this with. And (as stated before) your filtration would need to be massive. To have a pair of fx5's or 2180's on a 180 would probably be a minimum, and even then I would do water changes 50% w times a week. Good luck.