Yes - good info. I wouldn't mix bettas.
With black gravel some brightly colored shrimp would look great in there. Question is...will they become betta snacks? Before investing in some pretty shrimp, I would test out your bettas with some cheap ghost shrimp. There are some really pretty apple/mystery snails out there in shades of purple, fuscia, gold, blue, and ivory that would look pretty in there as well. Also to be tried with caution though. Some bettas can't resist to pester a big ol' snail.
With black gravel some brightly colored shrimp would look great in there. Question is...will they become betta snacks? Before investing in some pretty shrimp, I would test out your bettas with some cheap ghost shrimp. There are some really pretty apple/mystery snails out there in shades of purple, fuscia, gold, blue, and ivory that would look pretty in there as well. Also to be tried with caution though. Some bettas can't resist to pester a big ol' snail.