Well Im not 100% sure but, I think Mollies will out grow your tank. They look small, however they get big. Also they have lots of babies, so this may overstock your tank. Only if one survives every month, slowly you will be getting more fish added to you tank. Also your pleco is almost guaranteed to outgrow your tank. Which all isnt a good thing.
Definitely agreed. They may look small now, but mollies grow large. If you'd be willing to go up to 30 gallons, you may be able to keep them.
But, if your betta is getting stressed by their presents it might be time to return them. After all, each betta is an individual and some just don't tolerate tank mates.
How big are the molly now? Just curious. If they are larger then 2" each, then this may heighten the aggression for they feel cramped in the small 10 gallon.
If you really want to keep the mollies, then you could always get a 30 gallon for them and keep your betta in his 10.
BUT molly are livebearers and WILL reproduce like mad. You'll be overrun with 'em before you can blink....and IME molly don't always eat their young as other livebearers do. If your set on keeping them, have a plan for a LOT of fry or return any females
Glad I could help with the pictures
any pics of your guy? I'm a sucker for CTs, DTs, and HMs