Well there is a few things to take into account here
What type of "ramshorn" do you have? Is it the giant ramshorn species(apple snail)or is it the regular ramshorn species(planorbid, spelling may be off
Then Mystery snail doesn't say much. What colour is it? What shell type does it have. It could be a cana or brig. In either case no, they will not cross breed. The only hybrid I know of in apple snails is spixi/marisa(or giant ramshorn).
Oh and yes, apple snails poop a lot if healthily fed. If your not willing or able to upkeep it then you will need to bring it back. Depending on the ramshorn species you have she/he needs 2.5g or so, if you have a brig you need another 2.5g or so and if it's a cana you need another 5g or so.