I'm just like you in getting attached to animals. :)
Before you should add more fish you should try and cycle your tank. Cycling is establishing a cycle of bacteria that turns the toxic ammonia (which comes from the fishy poop lol) into nitrate which is much less toxic. It's not too tricky but it takes a while (about a month :$). You can do it with your betta in there! Here is a good thread on it: http://www.bettafish.com/showthread.php?t=47838
Your tank isn't large enough for any other algae eaters other than a nerite snail or otos. :(
Otos are quite fragile, so they usually like to be in a well established tank (up and running for over 6 months), so they probably won't be your best bet.
I love nerites. :) They eat algae like a champ, are small and don't breed in fresh water.
Apple snails get big and are messy. They poop TONS! lol They aren't very good algae eaters. :(
Another option is to clean of the algae yourself. I'm not sure about live plants - but when I get algae on my fake plants and caves I just take them out of the tank and rinse them under hot water. With a little scrubbing the algae comes right off. When it gets on the walls of the tank I use my gravel siphon to suck it up. :)
How long do you have the light on in the tank during the day? I'm guilty of having it on too long. :P In my experience, if I leave it on for 12 hours or more in one day algae will appear.
I hope that didn't sound too intimidating. :/ Fish-keeping is a blast and it's really rewarding.
Good Luck! :D