Snails are good companions for most bettas. The betta may initially flare and peck at the snails (usually harmlessly on the shell), but eventually will ignore it completely.
You may want to consider doing some additional water changes to control the algae though. While the large apple snails are helpful cleaners, they are not a cure-all, and some maintenance on your part to control nitrates is needed.
Without adequate algae, snails can starve, and this will pollute the tank. An algae tab is often too much for a single snail to consume quickly, so breaking a tab into fourths and providing one piece every 3-4 days will supplement thier diet. the betta may pick at the algae tab a bit, but this is unlikely to be enough to starve the snail. Likewise, the snail will happily eat any food the betta overlooks.