I have found that Selleys knead-it regular, not the waterproof stuff is best for small dings. It does not melt the stryro. I use it for repairs all the time and then finish them off with polyester resin (standard surfboard resin).

I reckon that all skin fittings going into Styrofoam should be set into some form of waterproof and tough type of material, not directly into the Styrofoam especially if they are going to be stressed by flexing . But deck plugs for a handle type thing it should be OK to just insert directly into the styro. But make sure it is a tight fit otherwise you will get a heat buildup and melt the styro. You can not control the epoxy hardening reaction by adding less hardener to slow down the mix and try to keep it cooler, the resin will not set.

Rod