Water quality is the main concern. Yes, young plecos can physically fit into a small tank, but if you take their impact on the bioload into account, they are already too big for the tank. Plecos...poop--pardon the wording--crap ton. Keeping a pleco in a 5 gallon tank, especially with other fish already in it, is going to be so much more trouble than it's worth. Imagine swimming around in toxic waste all the time. That is what it will be like for a pleco and all its tankmates, unless you are performing water changes every other day (or daily).
Also, plecos are actually territorial and like their space and hiding places. In a cramped tank, they can get stressed out. And that leads to illness and aggression problems. Plecos can attack, injure, and even kill other fish.
Plecos do grow quite fast. A healthy juvenile can grow 1-1.5 inches per month (no, it's not overnight, but they can surprise you), not counting sporadic growth spurts. So, you are looking at a pretty big fish fairly soon down the road. I have kept both plecos and cichlids and have experienced this first hand.
Last edited by Fenghuang; 05-10-2013 at 12:01 AM.