First, CONGRATS!

You can simply ask them which songs they were interested in.
I don't know if I'd call it stock music but it's RF and specializes in music for videos such as weddings, etc. I know some folks who made a few hundred dollars a quarter with them. I made much smaller amounts but didn't have many tracks signed. And I signed with them when they were non-exclusive. As with other similar libraries, it's possible that with the newer subscription model, the payouts are significantly less now.
Whether to sign or not depends on some factors. What else would you do as far as pitching these tracks if you didn't sign this deal? If they would only languish on your hard drive forever then it's a no-brainer to go for it. But if you feel the tracks can get better opportunities with more Film/TV broadcast oriented libraries, hold out. BUT keep in mind that instrumental cue libraries are getting more and more selective as their catalogs fill up. Do you write a lot of cues such that you could write many more in the same style(s)? If that's the case, sometimes it's worth putting a few where an opportunity arises.
Best of luck,
