Liz, it looks like you and I are on the same path. Not as far as puffers go, but as far as going from brackish to marine.
As for the damsels, they are the most territorial fish I've ever seen, and definitely think one could hold its own against your puffer, no questions asked.
I have two "freshwater" moray eels, and a "freshwater" toadfish. I plan on getting a lionfish (smaller one, though, since I'm limited on tank size) but I'm eager to see what you decide! Do you already buy your salt by the bucket? I believe if you go online you can find it for a pretty decent price! I've seen one person said they go to PetsMart.com and get a print out of the price online, and the store will price-match it, as long as you have the printed out page with you. It's something I am going to look into, and I think it could benefit you as well!
As for the puffer chewing on the silicone...Maybe this is a question for pufferpunk, but could you put in a cuttle bone similar to what they sell for birds? It's just calcium, and comes from cuttle fish, so I'd imagine it wouldn't be bad for your water, even help buffer it a bit? I'm just trying to think of ways to help curb his chewing behavior, and I know with dogs, it's a natural behavior, and the best way to control it is to give them something they are allowed to chew (i.e. natural uncooked bones, chew toys that can't be broken into small pieces/swallowed.) The cuttle bone was the first thing that popped into my head.