Your betta will love you for getting a heater! It made such a difference in the way my betta looked and felt.
In the bowl, I also used lights to keep him warm during the day. But once the lights were off at night, the temp. dropped rapidly. Bettas, like most creatures, need a cycle of day and night, so you should turn off the lights at night. But I realized this constant day/night temp. fluctuations were very bad for the betta. The heater will work whether the lights are on or not!
What size is your tank? I would be cautious about putting the 25-watt heater in anything smaller than a 5 gallon tank. I have seen smaller heaters at Wal-Mart, but don't know how safe they are. I've never seen anything smaller than a 25-watt at the pet shops.
I think BettaMax would be ok for the snails. The label says "safe for the community tank". Get a paper cup of tank water, open the capsule, empty the powder into the cup of tank water, let it dissolve (stir with a plastic spoon if necessary), and then pour it in the tank. Be careful -- the powder may stain your counter tops or other surfaces (and your fingers!) The green water doesn't seem to stain anything, though. Just wipe it up quickly if it spills.