Urchins will not go anywhere near corals (for food), they are algae eaters. There is a possibility that they might accidentally knock something over but that's about it. I would return the kenya tree, many people see them as weeds more than a nice coral. They grow very quickly and very easily will take over the tank. They reproduce by dropping their arms which will then float around and attach to another rock. I just returned two that i had in my nano. I thought if i kept them isolated they would not spread but once they started dropping their arms i was picking out probably about 10 chunks a day. They definitely had to go haha. The powder brown tang will also not live long in a 36 gal
corner tank. Tangs need a 6ft long tank for swimming room, if not they will get stressed and die eventually.
If you get any baby cardinals i would catch them and separate them from the other fish. If not they will be eaten. They spend awhile (i forget how many days..its probably in those articles) in the fathers mouth after hatching so i would catch the father a couple days after you noticed they have hatched and move him with the babies to another tank (that has been set up and cycled). Once he releases the babies you can move the father back to the display tank. The babies also love a place to hide in. I have read of people making their own replica of a long spined urchin and the young took to it right away. The babies will need to be fed live brine so you will need to get some eggs to hatch them. It's all in that first article i posted actually im pretty sure.
Good luck! and make sure to post if they ever decide to keep their eggs