Welcome to the hobby! You can certainly keep a Peacock in a 30 gallon, but they do like to wander a lot and might find that space a little cramped, especially if you get a 6" adult. My Peacock is in a 40B that has a nice footprint, but I still wish I had more room for him to explore. He explores every corner of the tank about 3-4 times per day at least.
While I can't tell you for certain how aggressive your individual mantis will be, I will tell you that you'll have more luck with the damsel than with the goby/pistol shrimp combo. In general, fish that swim up in the water column fare better with Peacocks than those that don't.
My Peacock absolutely hates anything it finds in the sand and will kill it immediately. If it sees a crab or something hiding under a rock, it will dig tirelessly until it flushes it out. I've never tried a fish but something tells me that it would be a no-go. I used to have cleaner shrimps and Peppermints that did ok. He killed every single one of them eventually.
Long story short, avoid the goby/pistol, but many (not all) Peacock owners report success with a damsel or other small fish. Best of luck!
