My only concern with using slugs as a feeder would be potentially carrying parasites; it might be best to wait until you have an F2 generation of slugs to separate from the primary group you collected to begin actually feeding to your frog. Then again, I don't know that much about slugs specifically, (more their cousins) - they may not be prone to carrying bad stuff within them. Honestly - a healthy animal should be able to combat anything that may present itself to begin with, but as a precaution for common practice in captivity, we usually opt to avoid feeding the unknown.

That being said - those slugs should reproduce for you "fairly" quickly, especially if you have so many of them at your immediate disposal. They are hermaphrodites, and if they are like some other gastropods they probably copulate to reproduce but are fully capable of reproducing with themselves to lay their eggs. I have no idea what species you have so I can't be certain as to how quickly they can reproduce, but mature garden slugs of the typical pest species can lay eggs every 2-3 months in varying quantites (often HEFTY quantities). The rate at which these eggs hatch can vary dramatically depending on how damp you keep them and the temperature you are keeping them in. Slugs like things on the cooler side [~65 degrees]; but eggs hatch faster if kept a little warmer.

Honestly, if you keep them damp and provide plenty of food they should breed on their own for you. I'd keep a constant supply of fresh salad items available, a good substrate, and some dead wood probably couldn't hurt to increase surface area and for them to munch on(nothing with excessive phenols). I think the setup that the OP has would probably be sufficient to culture them. Hopefully someone with actual slug experience can help, I'm just drawing my ideas from research on prevention in my garden and some snail stuff I'm familiar with.