20 gal

In a 55g yes but in a 20g all bets are off.

stick with the plan for the 55gal its a good one as for the 20gal if your peacocks are juvies right now like an inch or two should be ok for a bit but when around 3 to 4 inches one will prevail, good luck..

I would at least a 55 may be a 75 with a little greater surface area on the bottom of the tank so they have more room to spread out horizontially

I agree the 20G is ok short term but when those fish get a lil bigger two peacocks will likely have at each other until 1 is dead...matter of months I would think before you have serious problems. Reason being that you need to overstock these fellows to manage aggression and ya just can't do that in a 20G.

A standard 55 would make a great tank for africans - you would add a bunch more as per the above. A 75 would be stunning.

Other cichlids for a 20 - I presume not a tall/hex tank :

Pair of jewels - nothing else with them, have a plan for fry.
Trio of apistogramma species - and you can have some a group small schooling fish and say some corydoras as well
Pair of Rams - and as per the apistos for tank mates
Tanganyikans shell dwellers - see here for suggestions for that size tank www.cichlidforum.com in the library section. This would be a species tank.

for a 20g their are may dwarf cichlids you can look into. A pair of something small alone would be great just be prepared for babies if male and female and beware of aggression if same sex. U can also try just one dwarf cichlids a centerpiece fish and add some compatible peaceful fish in the tank.