I was going by AqAdvisor for the tetras, which says 5 minimum. I wouldn't put guppies in there. I think you want 14 gallons and up for them because they need a lot of space to dart around.
If you got 4 platies, your tank would be slightly overstocked, but if you were religious about water changes, that might be okay. Platies are bright and come in different colors.
I know I keep saying this, but if you just bumped it up two gallons and got a ten the 4 platies wouldn't be a problem and you could put 7 tetras in with a betta. Here's an example of a cheaper 10 gallon. In fact, Petco is having a $1 per gallon sale until the 15th, so you could get a ten dollar tank and buy your filter seperately. Anywho, this still illustrates the point that bigger doesn't mean more expensive.
One last thing, just because female bettas are able to exist in groups of their own type, it doesn't mean they are non-aggressive. You want to watch just as closely for aggressive behaviour from a female betta if you get one.
P.S. I apologize if there are some errors or confusion in this post. I currently have the mother of all headaches.