If you want snails I would go with a Nerite snail, they're smaller and have smaller bioloads than Apple/Mystery snails. Plus Ramshorn snails eat plants, no thanks. Nerite snails also can't breed in freshwater so you'll always just have one. :)
They feed off of algae mostly, so they'll do a better job of cleaning algae if you have any. Also pick up some algae wafers to feed your snail, be sure not to overfeed on algae wafer, supplement with blanching some spinach or zucchini, for that extra calcium to keep his shell extra tough!
If you want shrimps, that's a hit and miss. Most likely your Betta will kill them, so try to get cover for them, like some type of moss, or some hiding places. Ghost shrimps are scavengers they'll eat anything, leftover fish food, algae, etc. They're really cheap compared to other shrimps so I suggest them. Also with shrimps is that they molt, and during that them they're extreme vulnerable, a little poke from your fish could easily kill them. Supplement their diet with blanched vegetables too, spinach, zucchini, high in calcium stuff.
Also, no to the bristlenose pleco. They grow huge and you'll need a 20g minimum to house them.
Hope that helps.