It’s tough to say for sure, I’m thinking velvet... but you could treat for both just in case.
The scooter will need CP (chloroquine phosphate) which will treat both velvet and brook anyway. You don’t have any fish that are sensitive to it, so you could treat them all...
BUT, it’s not going to be easy to get quickly.. if you don’t already have it on hand :/
So, I think I would focus on the clown for now. Get maybe a 10-20g tank, powerhead, heater, and ammonia alert badge and move him over. You can use a sponge seeded with biospira to help combat ammonia. You’ll also need copper and metroplex.
Meanwhile, see if you can track down some CP. If you have a vet willing to write you a prescription, you can order it from diamondback drugs (it’s been verified 99%+ purity). Otherwise, national fish pharm recently started offering CP as well.
These links will get you started:
How to Quarantine
https://r.tapatalk.com/shareLink?share_fid=1020&share_tid=189815&url=https://www.reef2reef.com/index.php?threads/How-to-Quarantine.189815/&share_type=t
Copper treatment
https://r.tapatalk.com/shareLink?share_fid=1020&share_tid=193343&url=https://www.reef2reef.com/index.php?threads/Copper-treatment.193343/&share_type=t