A few suggestions...you can still use your current rock in your new tank, and adding more rock shouldn't be an issue at all. You can shake off as many bristle worms as possible to reduce the population, but the best way to reduce them is to feed less often (the amount you have is directly related to the amount of food available to them!)
When you set up your new tank, be sure to have flow behind your rock work (and throughout the tank in general) to reduce where detritus (and uneaten food) can get trapped.
You don't mention how old your 75g tank is -- many tanks do not develop coralline algae for many months. Many factors, such as your parameters, flow, how long the tank has been established, type of salt, if the rock has other nuisance algae growing on it... can affect the growth of desireable coralline algae.
If the rock you are looking to add comes out of an established tank..and you keep it submerged & at the correct temp and put it directly into your new tank, it should be fine. If you can't/aren't able to do so, then you can have a mini-cycle. Either way, you want to keep an eye out on your parameters.
When you do your upgrade, can your previous tank remain running for a time while the new tank (with the new rock in it) begins to get settled in on it's own?
Do you plan on moving your sandbed to the new tank? How old is it?