Hand-feeding fish is a lot of fun. I've got my goldfish doing it and I'm working on the koi. As far as the dangers of overfeeding there are some. Overfeeding in bettas (basically cousin to your dwarf gourami) can lead to buoyancy problems. Overfeeding in goldfish (not nearly as closely related) causes GI bloat. You'll know when you are underfeeding your fish as their tummies will look sunken. Depending on how big the little round belly is, you might be overfeeding. You should just barely be able to see a bulge. Looking at the profiles of the gouramis (including dwarf gourami) here should give you an idea of a perfectly-fed gourami.
This is an overfed betta. If your little guys stomachs look anything like this, you should fast them for a few days.