Sometimes they come in on a plant. It doesn't hurt anything. Just pick them out and throw them away when you see them. I had a few and then they just stopped. If you get an infestation there is something you can get but I don't even remember what it is called because I never needed it.
I doubt my catfish will eat them, but maybe my frogs will (since supposedly they eat Anything that fits in their mouth) But I am not sure I want to add loaches to a 10 gallon. I did hear that about them tho.
Just a suggestion, the pellets are constipating for Bettas even though they do make their colors stand out better. You may not want to give more than 2 pellets a day and supplement with freeze-dried bloodworms (5 or 6 in a day). They will try to make you feel guilty and like you are starving them, but believe me the problems of an overfed or constipated Betta are not problems you want to have to handle. The little gluttons will snatch them up as fast as they hit the water so you shouldn't have to worry about overfeeding if you cut back to that amount. The other worms may be the source of the snail if they are fresh or frozen. The eggs could easily have come in with live food.
SO far I can only see one (thank god) and I am waiting to see if my frogs will eat it. But it is VERY tiny and a kind of bland red color. It has the big slimey body with the tiny slimey head and those little antenna like things coming off his head.. lol