Depending on filtration, you could have two or three African Dwarf Frogs. Their bioload is about that of a large Tetra. You'd need places for them to hide; a terra cotta pot works. Sand is better substrate as they they are practically blind and could mistake a piece of gravel for food.
I do not recommend shrimp in such a small tank unless it is heavily planted. In addition, chances are you will lose the shrimp as your tank is not cycled.
I would get the Betta first; check out his temperament and cycle the tank. When and if you add tank mates all you need to do is turn the lights off while acclimating and leave them off for an hour or so after you release.
I've never had problems with the Betta being the first resident. If the Betta is aggressive and won't tolerate tank mates it doesn't really matter which is introduced first.
I also do not recommend new keepers add tank mates until an aquarium has been fully cycled for a minimum of a month; two is better. There are so many subtle signs something is going wrong that new keepers might not notice until it's too late.
This is excellent reading:
http://www.bettafish.com/showthread.php?t=555434
