I say 3 more. That's pushing it following the inch per gallon rule, but as long as you watch the water quality, you'll be fine. Especially with platies. They're smaller. Mollies, platies, and guppies are good tank mates, so any combination is fine. Just make sure you add the same sex mollies or platies. I mean each species, not all female to match your guppies. You can get male platies or mollies. Just get only male. 5g just isn't enough space to add fry to
And if you do add male mollies, they can successfully mate with your guppies. I don't think it commonly succeeds and most fry are not viable. Viable fry are sterile, if you do get a few. Platies and guppies cannot mate.
I would add 1 black balloon belly male molly, and a red platy, and one more of whichever looks better at the petstore to add some color since your guppies are females. Maybe a gold or a gold dalmation balloon belly- males. I like the male balloon belly mollies best. They are very cute and stay smaller than females. Enjoy
PS Love the color of the betta on your avatar. Gorgeous!