Hm i would also say no to the betta, i could see the angelfish and the betta having a lot of issues with each other. You could have 20platies, but in an all-female or mixed gender group you would definately get a lot of fry (which you need to either rehome or cull, as you wouldn't be able to raise them all considering how many fry female platys can give birth to on a regular basis).
How about some bottom dwelling fish though? Personally i'd go for a sand substrate as having a sand instead of gravel substrate will open up a lot more stocking options when it comes to many bottom dwelling fish like all kinds of loaches and catfish etc.