I wasn't worried about the baby issues, I was more worried about that large chunk taken from Suvi's tail which looks a lot like Betta damage. Livebearers of all types can get semi-aggressive with each other but it's usually more if it's a bunch of males together and one female and it isn't usually this bad either. I would keep a very close eye on your Betta boy in case he decides he wants to snack on more finnage from your Platies.
Usually when you want to put a Betta and other fish together you want to introduce the other fish first and the Betta last so he doesn't have a "territory" and the other fish aren't intruding on his territory of which he will protect; which can result in injury, stress and occasionally death if they are really into it. So I would just watch them very carefully, do frequent water changes so you can help keep Suvi's tail clean of infection.
@Kiara, all livebearers have a large bioload, they are constantly pooping and when they aren't pooping, they are eating and picking at algae. They don't have as large of a bioload as algae eaters like Pleco's and Oto's I don't believe but it's much larger than any Betta. Generally the filters will be fine but there will still be poop there which can just be annoying IMO is all. I know not everyone cares about that side of it though, but yes they do have a fairly large amount of bioload comparatively.