Zelfin,
Some people would say that your tank is way overstoked for anything, but definately for a reef (ie - only a sailfin tang in a 75 gallon, as they will grow pretty big, remove all other fish except the Goby). But I do not know enough about the other fish to offer a good opinion on this.
As for converting it to a reef, (WATCH OUT - completely addictive!) Angels have a bad habit of nipping on soft corals, so those would have to be watched and maybe removed (depending on what you were keeping).
Assuming you have adequate lighting, the main problem you will have is that coral is a lot less tolerant of water chemistry issues than fish. If your water parameters get out of whack, your coral will suffer. With that many fish, your bio-load may be causing water chemistry issues, that may not affect the fish, but will affect the corals. Unfortunately, stocking lists for a reef tank are generally a lot lower than those for a FOWLR tank.
So I would beef up your filtration (more live rock, and a good skimmer if you do not have one), and remove some fish. As for removing the fish, while people always say "I will take them out if they get too big" there are two problems with this:
(i) by the time you realize they are too big, the fish are already completely stressed out, so it is in many ways too late,
(ii) you have a reef tank, with sensitive coral everywhere, that does not like to be touched and/or moved...so removal becomes "difficult".
HTH
Stewart