I wouldn’t suggest any other fish with a betta in a 10g. They really need a heavily planted 20 gallon long minimum so that if anything goes wrong the fish can get away from eachother. It’s also very hit and miss with bettas, there’s no guarantee they’ll stay tolerant of the fish long term either.
I’d suggest either leaving it as is with the frogs or getting a snail or two and calling it quits then. You could also try a divided same gender betta tank, but that does take a few extra percautions as well. Harlequins would also need a 20 gallon minimum.
There's a lack of space in a 10 gallon to keep a betta with any other fish. It's too risky.
I think it depends on the betta fish. I have one in a 5.5 g with 2 feeder guppies and I haven't had any issues with them all. But it's a female and they are less aggressive than males. I think you would need to try and find a not so aggressive betta if you wanted to keep any tank mates with them.