Personally, I think that he'd be fine alone, but if you wanted you could get tank mates. There are a whole lot of possibilities, but you'd first want to cycle your tank
. Since you only have one boy, you could do a fish in cycle if you're careful. I would simply recommend that he is the only fish in the tank until it is cycled, that way ammonia doesn't rise too fast and make him ill. Just do a 25 to 50% water change once a week. Over the next month or two, the tank will slowly cycle.
I noticed that you didn't list a dechlorinator. Prime by Seachem is an excellent and economical choice, because the dosage is just 2 drops per gallon :D. Oh, and I don't think you'll need a gravel vaccum/cleaner simply because the sand is so fine it'd all be sucked out. You'd be better of getting a small siphon and simply running your hands through the sand to shift it around a bit and get the mulm in the water before getting the siphon going and just removing the water.
When it is cycled, you'd be able to go with a lot of choices. In a ten gallon, you could go with one of these groups: a group of 6 kuhli loaches would get on great with him. If not the loaches, you could do a group of four platies, or a group of 6 endler's livebearers. Since shrimps have a very low bioload, you could probably add three or four to the tank in addition to one group of whatever other fish you want. However, you'd just want to be warned that some bettas do enjoy midnight shrimp cocktail snacks. Or, if none of those strike your fancy, you could divide the tank and have another betta boy in there. :D
In the five gallon, you'd be limited to shrimps and snails.
When your tank is cycled and you're ready to introduce whatever tankmates you decide to get, you're going to want to remove your boy, rearrange the tank somewhat, add the new fish (giving them time to explore and relax), and lastly add your betta boy. This'll minimize possible agression. Of course, how well the fish get along will mostly depend on what your betta's personality is like.