first, how many do you have as juvis? and 2nd depeinding on the type of 20 gallon it is, you can either have a pair with no other fish or a male or 2 females with a small schooling type of fish, like danios or tiger barbs or some type of larger bodied tetra.
its a 20g standard. 3 big rocks, and some plants inside. am getting 4 juveniles. was thinking either blue rams, or a couple of green tiger barbs and a couple of kuhli loaches
no, 4 cons will not get along if one pairs up. if theres a pair then thats all that will fit in a 20 gallon. trust me, a pair of cons can kill ANYthing they set thee minds to.. my pair i had to divide cause they were pickiing on my oscars and this is a 112 gallon. they coulda killed my oscars which are 3x there size.