I wouldn't worry about him pushing the rocks around. That is probably a good thing. He can dig up food and escape if there is a mishap. I have pvc plumbing elbows in my setup. The frogs prefer to hid under them rather than in them most of the time. I don't mind, it's their home.
As far as another frog, I would first upgrade to a bigger tank, then use the 10 gallon to grow out any frogletts you may buy. I find that my 1 and only male is much more shy than the 2 females. He is also tiny compared to them though. He will come up to eat, but only if it is dark in the room. Feed only every other day to every third day. Try feeding at night and turn the filter off for 2 hours. After that remove uneaten food and turn the filter back on. He will catch on. You have plenty of time too as they live a long time and can last a little while without eating.