How big is the harlequin? Like I said it is a recomendation and does not mean it cannot be done. If you keep the harlequin then I would skip the royal gramma and just add the clown. Maybe some type of goby down the road.
From what you are telling me you have about 80 total watts of light on your 20G? That should be enough for just about all the soft corals and LPS
hard corals. I would stay away from the SPS
Hard corals as they are usually for the more experienced. Take a look at "myinfo" just about everything I have would do well under your lighting.
As for how many you just need to leave some room between them so they cannot touch each other. Usually 3-6" depending on the coral. Be advised that some have sweeper tenacles that can reach out a few inches. Also, some like you mushrooms will spread on their own if they are doing well.
and polyps you have should be doing well. I would also look at different varities of zoos and button ployps, Leather toadstool, candy cane, pulsing xenia and devils hand which are all easy care level.
Here is a link to give you some idea of what is out there.
Again, just space them so they have some room to spread and grow. Again you may find one that tries to take over the tank and will need to be harvested. Some people use a 20g long as a propegation tank for coral frags. Start small as they will grow.
Before you add any more, what kind of equipment are you using (i.e. filtration, skimmer etc.)?