As it stands right now my rics are dividing faster than my regular mushrooms are multiplying.. One steel blue ric I have divided into 4 rics total in under two months, where as the mushrooms have only grown in size, and not multiplied once in the past two months. A lot has to do with how much they like your tank, I see you have a 2 bulb T5 fixture on a 55, so they might like the lower light a little more. I'm running a 4 bulb t5 fixture on a 10 gal. If they spread, you can always cut them off close to the rock, they usually regrow from the "stump" but it takes time. My mushrooms have pretty much stuck together, the only exception being one that didn't like its positioning. It let go and floated around the tank for a few days until it settled.