The snails shouldn't be a problem, BUT they will reproduce incredibly fast, so most people view them as a menace. It's really hard to get rid of them in the end as well. They can end up making your tank look less attractive quite easily with all their slimy eggs and stuff d: So in other words, they ARE a problem. Not necessary for the filter.

Anyway, for what female's sake? Is it a female guppy you've got? In that case I'd recommend you not getting any more guppies, as they will reproduce very fast.  Even if you'd buy another female, she could've stored the sperm of another male inside, and might give birth to more guppies on her own, meaning, more fish, more fish and more fish.
If it's a male guppy you've got, feel free to buy one or two more!

andiy: It's okay to keep two males if you keep them separated, hence "I currently have a ten gallon divided tank...housing two male bettas"
I don't know if bettas will for sure attack guppies though, it has crossed my mind whether it's a good idea for me to put these fish together myself, but as far as I know, bettas attack fish of similar body type and size. They also don't hesitate to attack fish of the same color as itself.

When I first got my tropical tank up and running I bought 1 gourami and a few zebra danios and 1 female guppy.
Well those darn zebra danios ate the guppy and pecked one of the eyes out of my gourami.  They are so hyper and I think they are really nippy!