My male VT Kudo is in a 10 gallon tank with his new tank mate Gus a mystery snail. It all depends on the betta, some are more aggressive then others. Kudo has tolerated tetra and ADF in his tank, my new super delta Spyro I think will not tolerate any tank mates, he's more aggressive. But to answer your question, I think a snail would do wonderfully with a betta in a 5 gallon, I've learned though through this site not to overcrowd due to bioload issues so just be careful how many tank mates you get!
If you always do what you've always done, then you'll always get what you've always got.