The clowns will definitely hurt the goni. By simply hosting in the coral, it will be under stress and won't do well. As winy pointed out, the goni is hard to keep anyway. The people who seam to do the best with gonis report a LOT of feeding. Apparently, they are a high metabolism coral and need to eat a lot. But thats just what I've read. I've never actually had one.
How to stop the clowns from hosting?
I had to train my little clown to stop hosting in my acans. It took about 2 or 3 weeks of CONSTANTLY swatting at him with the net every time I caught him in the acans. The acans would shrivel up and retract when he was hosting. They weren't getting enough food or light. I must have swatted at him 100 times a day for a couple weeks. I finally trained him to leave them alone, but it was very frustrating.
Emerald crab:
They are usually pretty peaceful and will eat algae when they are young and small. But my experience with larger emeralds is they are just like any other crab. Opportunistic predators that will munch on anything they can get their claws on. That includes corals, but it's rare. They'll eat corals if they don't have bristle worms or snails to attack. Caught mine attacking a nessarius snail and a bristle worm one day. I finally gave up on emerald crabs for algae control. They don't eat enough of it to do any kind of real control. They get big and turn into predators, just like any other crab. They kill each other. Thats not what you read on all the sites like Foster Smith or other online retailers, but trust me - you put 2 different sized emeralds in the same tank - the little one will be dead in less than a week.
Just my experiences in a 30g tank. All tanks are different and individual ecosystems. All animals are going to adapt and act differently in each different tank.
