I had an 8 inch bristleworm in my tank once. They are good for your tank (and I can guarantee that you'll have some in your live rock) but when they get to be about 6 inches long, I try to take them out because they look kinda gross :(
Purple Up won't work without the correct lighting (since it promotes the growth of coralline algae), and if you are doing large water changes while your rock cures it would be a waste to add all those chemicals. In fact, it is debated whether those chemicals are needed at all for any tanks. Some people say that regular water changes will replenish any elements you need in your tank. It is also possible to overdose on those additives.
I'd recommend that you not put anything in your tank that you can't test for. That way you will know if you are adding too little or too much, and too much can hurt your tank inhabitants. Calcium is definitely important, and if you add it to your tank you need a calcium test, but that is the only additive I put in my tank.