That would probably be overstocked/poorly stocked even without the fry... Typically angelfish alone need larger tanks than that.
Generally when breeding fish you set up a species only tank, something similar (such as a breeding tank), or a tank with the focus on one species but containing other fish that will not be threats.
I think if you want to breed fish you should pick which ones you enjoy the most, create a set up suitable to their needs, and go from there. I honestly do not know a thing about angelfish or ram breeding and very little about plecos, but I do know that you will have a hard time keeping, and raising angels in a 20.
Even if you somehow did get all three species to breed in the same tank the babies would never last with the other fish in there. Remember- big fish eat other fish that fit in their mouths. Almost always.
If you choose what species you like the most, we can help point you in the right direction.