I wish I could remember who to give proper credit to, but my favorite quote about this type of thing is, "The fish never read the books."
If those two clowns are getting along, then let them be. If they start to fight, you'll have to choose between the two and trap one out. Clowns are known to be brutally territorial, but if your tank has only been set up for only a couple of months and your Maroon is young maybe he just hasn't set up territory enough to have a problem with the other. Clowns don't tend to claim a big territory, so maybe you'll be lucky and they'll both just take spots on opposite ends of your tank.
I don't know about the pin cushion urchins. When my tank was new I had a LOT of black urchins as hitchhikers. Others convinced me to get them all out because they were supposed to eat coralline algae, which is desireable in a new tank. They did grow alarmingly fast, so I pulled them all out.
Now that my tank is 2 1/2 years old and keeping the coralline under control is the biggest pain I have, I sort of wish I had some urchins if they really do eat coralline. It would be my luck that they'd eat the coralline off of the rocks where I want it and not the sides of the tank where I don't.
Good Luck!
Cathy