Ah, the old every fish is like my mbuna trap. I've seen a few people fall into it. Welcome to that club.
Guppies are hardy and their behaviour is very stereotyped. If they're fancy guppies they'll swim around having fins, and that's about it. If they overload the tank, they'll be smaller and live shorter lives. Do enough water changes, and even a beginner can do that.
But why?
Overstocked non-mbuna tanks are boring. The fish are just ornaments, not living creatures acting in ways that are fun to watch.. You get no complex behaviour, just a permanent subway platform at rush hour. Everyone there is nicely dressed, and that's about it. The fish are stressed and disease prone, and live unpleasant lives. Eventually, the tank crashes and you do it all again with more unlucky fish.
When it comes to moving past the beginner stage in fishkeeping, non mbuna overstocking is underappreciating. You've put the plan to overstock first, even before you've decided what fish to apply it to.
You have a 20 long, and you want lots of fish. You don't seem to want to breed the fish. Why not raise your game, stock reasonably but go for micro fish? With a little planting, you could have a big group (easily 30) of Bororas in there. You could find Heterandria formosa, and really go to town. They thrive in large groups, and stay small. You started with the only group of Cichlids that like to be crowded. For your tank size, look for micro fish that live in large groups if you want to keep moving in that direction. Chili rasbora are a common Bororas species that's generally available, likes big groups and won't break the bank. Plus they're active, social and fun to watch.