depending on the type of pleco you want to add... but honestly, plecos get lazy when they get bigger (unless they are bristol nose plecos) so adding a small one does more for your tank.

in a community tank... i recommend using ottos or flying foxes instead. they loooove to eat algae and don't add the same amount of poop as your pleco will.

that said, in my 210 i probably have at least 40 plecos... i collect them so i'm a hard core pleco lover

the reason many of the livebreeders are underrated is because they are so inbred so often they aren't all that hardy. but if you can get some good healthy ones... they are a great fish

oh and as for lighting... fish need a day and night cycle. turning off the lights cause you aren't there is not recommended (sorry emily). so unless you have a real problem in the tank (medications which need the lights off or overgrowing with algae and you need to get it under control) don't have the lights on a shorter cycle than 10 hours on.