It's possible your Betta may nip at your frogs, but I doubt he will. Most of mine have been quite gentle with my frogs, and will actually let the frogs hitch a ride around the tank (yeah, like the Betta has a choice....). Expect some of the ghost shrimp to go missing, though, if he gets hungry.
I'd go with a 25-50 watt heater. I'd be more inclined towards the 25, since the tank won't QUITE be a 5 gallon tank, but a 50 would probably last longer.
As for water changes, do a daily or every other day spot check to make sure there's no uneaten food (though with ghost shrimp, there probably will never be anything left uneaten long) and waste. It should be good to do a 50% water change with a good siphoning and waste cleanup about once a week, with a good, thorough 100% water change once every two weeks. More if it becomes necessary.
Just watch the frogs, especially during the 100% water changes. I've found the best method to catch them is with a net, and then clasp your hand over the net the very second it leaves the water, and have a small cup or bowl with a secure lid waiting for each individual frog.
