The only problem I see is that the tang and clown are pretty territorial once established. I'm a fan of wrasses so I'll focus on those. Some species of Fairy and flasher wrasses can be pretty timid and can die pretty easily from aggression; research for a boisterous species. I would suggest looking into almost any of the Halichoeres spp. They are generally more hardy, some will eat flatworms and nudibranchs and include my favorite fish, Halichoeres melanurus. H. melanurus isn't available often in Canada and you'll have to know how to recognize it (especially as an individual), but I didn't let that stop me! I think J&L has one in stock as well btw
. H. iridis and H. chrysus would also be excellent choices and are more commonly available. For a wrasse that will do an even better job of taking care of itself (but might further limit future additions), look into a sixline wrasse; occasionally a flatworm eater as well. THE biggest factor for wrasses though is a cover or canopy; they are all jumpers (no exceptions imo), so if you don't have a lid/canopy, forget them... then again, so are watchman gobies, so you should already have one.
Whatever you choose, you'll have to pick something that can take good care of itself and should not be shy about eating prepared foods. dottybacks and pseudochromis could be worth a look into; I've always been a big fan of orchid dottybacks and splendid pseudochromis. how about a royal gramma?
no angels since they put your coral at risk; the dwarfs will give you a 50/50 chance at best. Are watchman gobies obligatory sand sifters (I seem to recall they are, but I might be wrong)? If so, then I would advise against any more sand sifters due to competition for food in the sand since they might starve each other. Especially if this tank hasn't been set up for a good amount of time.
also, since no one else has mentioned it, I hope you have a bigger home planned for when that tang outgrows the 60g.