Though a limited amount of people have managed to have SAPs and dwarfs live together, it definatly isn't recommened. It more often than not ends up in either species attacking the other.
Honestly though, you won't regret making the tank a 1 species only tank. Puffers are fully of personalty and I'm certain you'll get attached to them in no time.
Whether SAPs or dwarf will be more for you, I can't tell ... there's good and bad things about both.
In a 20G you'd technically be able to keep 4 dwarf puffers, however, them being very aggressive little things, it often doesn't work out that way. When young they tend to get along really well, but after a period of months till years they often get aggressive and fall out. Though you'll absolutely love dwarfs for the way they'll interact with you, I recommend you only get them if you're willing to keep a couple of 5Gs behind, incase they do fall out with eachother and you need to pull 1 out so noone will get hurt. You might end up having 4 living together happily, or you might end up having to pull them all apart because they just don't want to accept eachother ... you just don't know until you try.
With SAPs most people tend to have more luck in keeping them together. They're 1 of the more laid back species of puffer, and will often tolerate eachothers company. I'd say that in a 20G, you could keep 4 of them as well (it's either or, not 4 & 4, but to make sure you won't read that wrong =)). However, SAPs are known for having very fast growing teeth. When owning South Americans it's pretty much certain you're gonna have to trim them teeth at least once in their lifetime, and if you're unlucky once every month. Trimming a puffers teeth is not a fun experience, and you've gotta be up for it ... besides that, SAPs too have a lovely personality, and are definatly very enjoyable =)
