Ramshorns don't have very high bioloads. If they did the 50 or so I have in my 10 gallon would have my ammonia off the charts. Apple snails have high bioloads but snails like MTS, Rams, Nerites, etc have relatively low bioload... however with snails it depends on whether you are over feeding or not.
To answer the OP's question, no, snails and shrimp will not have any issues if you put them together. They'll just leave each other alone. A well planted (almost full of plants) 2.5 gallon tank could easily support about 10 ghost or Red Cherry Shrimp adults and 3-4 Ramshorn snails. Just make sure you're regularly taking out babies so the tank doesn't become over populated. Weekly 50% water changes will keep your water quality good IME as long as you are feeding sparingly (every other day to every 3 days).
However, until you upgrade and remove the betta I would keep the stocking level where it is, just the snails. Even with java moss a 2.5 gallon isn't much of a foot print for shrimp to hide in.
RCS are easy to care for, however their red coloration makes them subject to predation because they stand out easily (especially if you have dark colored substrate). Ghost shrimp (IME) tend to last better against bettas. Also ghosties get a bit bigger so it's harder for the betta to eat the adults.
Thank you for answering my question!
Well, I already have a small bowl of Ramshorns, trying to breed them, and was only going to put the very biggest snails in the betta tank, and right now thats only one, but i havent put him in the tank, cause hes carrying the eggs on his back, instead of laying them on a leaf or the glass!
And I was going to set up another larger bowl for the shrimp, and then only take the few largest ones into the betta tank. And if they breed in there, the betta can eat the babies to its hearts content, since there should be the adults still there. And My betta is smaller, so i think theyd be too big to munch on. And for Christmas, my "Fishmas", I asked for all fish stuff, so i should be able to get some plants, which Im really excited to have!
And they should be eating the bettas poops too right? Or do I have to get algae wafers? I feed the snails a leaf of greens, spinach or cabbage, or lettuce, and a few flakes of the kois food, cause ive read it helps them breed faster, and they seem to love the crisps! You can tell which ones eat the crisps, cause thier poop is hot pink, not brown..lol