I have to echo the advice that many wise fishlore vets gave me - sharks need a minimum 55 gallons to give yourself a good chance of peace.
I disregarded this advice (well I tried rehoming, with no success) and kept a rainbow shark in a 36 gallon. I didn't have any tragedies, but he got aggressive with a DG he had grown up with and I finally pulled him out. He's currently in a decorated quarantine tank, but I caved and bought a 55 gallon to rehome him into myself. This is NOT the road I would recommend!!
Long story short: skip the shark, save the pain.
Gourami can be a little aggressive. I don't know a lot about how big blue gourami get, part of me wants to say 5" but someone can correct this. I honestly love them though.
Tiger barbs are nippy to say the least. Everyone agrees on that. Mostly depends on what other tank mates you want.
Corys are a great idea. I think pygmy corys are wonderful and they're tiny. Pandas I know also are small. You may want to consider sand for them, but it's debated that they can live happily on smooth gravel too.
I see you already have a few schools of tetras, but I would recommend finding a new school you'd be interested in having that's around 2". Then definitely corys and either a small school of top swimmers or a small feature fish. Some people don't like dwarf gouramis, and they can be prone to disease, but I really love mine and I think she has a wonderful personality. She's bossy but never did more than give a split second chase of one of my barbs.
Hope all of this is helpful! (and accurate - if not, I'm sure someone will correct me)
Best of luck!!
Cories are awesome bottom dwellers. I love mine. I think the red tail shark is also a bottom dweller. Not sure if they would get along. Not to be a downer, but I had a three spot gourami that I had to rehome because it was such a bully. And tiger barbs can also be nasty fin nippers. But this is just from my experience. Wait and see what you hear from more experienced people on the forum.
ok, 1 gourami, school of some sort of cories (depending on what my lfs sells) and tetras of some sort, maybe 7 tiger barbs. will cherry barbs work? does this sound fully stocked? still not sure on the cories though. idk if my lfs sells them.
Tigers are more docile if in larger schools... but I digress
And you could actually do otos with cories since cries are scavamgers and otos will eat any algae in your tank... plus (at least mine did) the cories can teach the otos how to eat algae wafers!