If the plug has a small piece that sticks down, try and break that off. With the ORA corals, I had to use a dremel to saw off the bottom portion of the plug.
I usually keep the frag on the plug because I like to keep it with the encrusted part on the plug. I find that the frag will grow faster if it stays on the plug and doesnt have to re-encrust an area where it is attached.
I use reef safe epoxy along with super glue gel. I put some super glue under the plug piece which helps it stick to the epoxy. Then I put some glue on the bottom of the epoxy which helps it stick to the rock. The super glue dries quickly which then allows the epoxy to harden slower. Eventually the sps will encrust over the epoxy or coralline will cover it and so you dont see the epoxy.