Yes, Malaysian Livebearing Snails will eat fish food that falls and sits on the substrate. So will pond snails and bladder snails. I feed flake/pellet food for the upper fish and sinking tabs/pellets/disks for the substrate feeders, and in tanks with only a few of the latter the disk might not get completely eaten, and I always see it covered by these snails so they clearly do eat fish food that gets down there. I have even seen the snails and fish chomping together on the same disk.
Algae will not contribute to ammonia; like higher plants, algae feeds on nutrients including ammonia and produces oxygen, though this is obviously much less than will be the case with higher plants that have a higher need for nutrients. The small snails mentioned above will eat algae as they find it browsing the surfaces, but even with many snails this will not be sufficient to deal with "problem" algae. But it all helps.
It is a good idea to run a sponge-type scraper over the inside of the front glass at every water change whether you see any algae on the glass or not. Surfaces under water develop a biofilm and algae readily grows on these. Cleaning the glass at every water change will catch the beginnings of any algae before you can even see it.