your 2 balas will outgrow your 20gal . if you leave them in the smaller tank there growth will be stunted and thats not the best for there longevity. they will be ok in there for a while, but its not true that you have to have 75gal each. they are very fast swimmers so a longer tank is the best for them.
as far as ur stock being compatible im not real sure, iv never seen, or had results that my balas attaked any other fish in my tank(i now have larger tankmates with them)
75gal 2 bala sharks,1glass catfish,3clown loaches 1of each,blue gourami,sun catfish,banjo catfish,giant asian upside down catfish,hillstream loach-10gal 6glowfish/blacklite