Are you wanting to do african cichilds or sa/ca cichlids? For SA, a pair of german blue rams could work well. They are fairly easy to sex. Gold rams or electric blue are harder to tell apart because of the coloration but can still be figured out. I would probably not do Bolivians in a 20 long, but they could work for a while. They tend to push each other around vs. actual biting/damage when fighting. Skip angels in a 20 long as it is too shallow for their fins as they get bigger.
If you wanted to try CA cichilds, a pair of rainbow cichlids might work for a year or more. Maybe some honduran red points, but you need a calm pair - I've had them kill each other in a 15 long (24x12) or even attempt it in a 20 long (30x12).
Or, if you go african, maybe a young pair of brichardi/helianthus/daffodils, but you would need to remove fry at some point so they don't overpopulate. Or julidochromis - probably the smaller ones like dickfeldi or ornatus. Or if you want some for life, try shell dwellers - multifasciatus (not as colorful but interesting). The problem with many of those is they are hard to sex based on external differences. So if you want something easy to breed, you have to buy a proven pair, or grow up 4-6, or pick a species that looks different male to female. Also they may not do well in a planted tank since they like to dig.
If you want planted, rams would be a great idea, and you could get a nice pair of german blues for $15 or so. Plus if they are tank raised vs. wild caught, they should spawn and hatch their eggs in regular tap water without hassling with RO and the like.
For plecos, in my opinion you need small plecos like clowns, bristlenose etc if you want cheaper ones, or some of the smaller ancistrus/hypancistrus if you go more expensive. If you go clowns, you need very good filtration to keep up with their waste, eg. an Aquaclear 50/70 even though it's only ~18 gallons of water, since they eat wood and produce lots of waste. Bristlenose would do OK with less vigorous filtration. They don't produce as much waste but still a decent amount.
Keep in mind that keeping plecos + cichilds means one or the other may not really spawn all that well - e.g. plecos eat the cichlid eggs, or cichilds eat the pleco fry.
