Couple of things.
I'd cut back to 1 ocellaris..... 20g is a little small for two. General rule is a minimum of 20 for the first one, and an additional 10 gallons for the second.
For the pistol/goby, you'll be OK as long as the tank is a 20 long, not tall. Minimum of 30g is recommended for most gobys, but I think a 20 long is plenty of footprint for the smaller species. Make sure your pistol is kept fed..... anecdotally, pistols are commonly kept with other shrimp, but if they are hungry, the other shrimp can potentially become prey. Other than that, just make sure your rockwork is setting on the bottom of the tank, since the pistol will be digging.... if it undermines a rock and everything shifts, it can collapse on the shrimp & kill it.
If you do get them, add the pistol/goby pair before the ocellaris to let the goby establish it's territory first, otherwise the clown may be aggressive, and the goby won't eat & starve.
I'd personally probably add a few astrea snails for glass & rock cleaning after the tank is established.........