IMO - -the clown and goby are just fine....the Hippo tang will get a little large for a 37 gallon tank especially if you have a lot of live rock.......
just a note to all who read this post....we need to remember that even a huge tank ....say 150 gallons with an immense amount of live rock in it limits the swimming area for our little freinds
back to your question........I don't follow the 2-3 inch per gallon rule .......i try to keep fish that look natural in my tank .......remebering the fish will "take over" an area in there as their own .......i would keep 4-6 fish in a tank your size all between the sizes of your goby and clown
there was a post on here where a guy photoshoped a huge discus into a little tiny nano tank.....i think of it everytime i want to try and cram something large or that will get large into my 20 gallon long
these are just my opinions