your shrimp and crabs are as good as food with the GSP im afraid... also your guppies could start to 'vanish'. its all part of keeping puffers.... no inverts or slow moving fish... or fish in general sometimes it all depends on the the temperment of your puffer.
you dont have tankmates with pufferfish, just food that hasnt been eaten yet. so dont get too attached to anything. but if you keep your puffer happy, well fed, and give him loads of things to swim round, you'll be fine.
rearranging the decor in the tank once in a while works wonders...
Yes, I once had a green spotted puffer, and he ended up with a tank to himself. He didn't eat all of my other fish, I just wasn't enjoying his temperment with the others. How big is yours right now? Mine was only about an inch, and dominated everyone. He did however stay away from ghost shrimp which was very odd... Best of luck, and the tank looks great!!