i wouldn't put a snail or shrimp in with a betta. the shrimp would likely become snacks and the snail would put out too much waste.
it's not 100% necessary, but i'd highly recommend flat glass marbles for the substrata instead of gravel. when we had gravel, every betta got fin rot, but since we went to marbles no bettas that we've gotten have got fin rot. The marbles do need a little more cleaning, but you don't have that gravel dust to deal with.
i'm not real familiar with that filter, but with most filters you should be able to get some sponge at the pet store that you could cut to fit into your filter. 40gph may be too strong for a betta in that size tank -- is it adjustable? If not, a filter like:
http://www.drsfostersmith.com/produc...004+22768+2032
might be just right for that size tank. It's good for up to 5g and is actually doing a good job on a 5g here. The flow is adjustable and it has space for a sponge.
If you go with fake plants - go with silk plants. Plastic ones tear betta fins.
http://www.drsfostersmith.com/Produc...&N=2004+113149
We get our silk plants from this brand name and our bettas love the broad leaved plants.
Best of luck with your Betta.. they are really wonderful fish who will show you a lot of character. They love to be talked to and decorations like small caves or bridges (check for anything that could snag their delicate fins) are a hit with them.
we feed our bettas:
http://www.drsfostersmith.com/Produc...30&N=2004+6130
and hikari brand frozen bloodworm cubes. Plus 1 day a week we feed them thawed frozen peas. If you'd like to ask more about their feeding, feel free to PM us or post questions in the Betta forum and you'll get quite a few replies.