Ditto to what Lilysmommy said. Mid-300s isn't automatically underweight - plenty of hedgehogs plateau around there. It all depends on the individual bodytype. The most obvious indication would be if either of them feel or look sunken in at the sides. Definitely get them on some better food though, it's about time they were getting fed well.
For the nails, do the best you can, but if they're really that bad and curled into his feet, you'd be better off going to a vet to get them clipped - especially since he doesn't know you and probably won't be very cooperative, and it can be really difficult to get a good angle when they're that bad. My suggestion for trying to trim them yourself would be to see if you can snip a little at a time, inside of trying to clip at the "correct length" all at once. Once you get them a little shorter (comparatively) you'll be able to see what you're doing better and have more room to maneuver the clippers, whereas when they're badly curled it would be difficult to cut a large chunk of the nail off in one snip. Hopefully that makes sense. Good luck!