Stocking Plan 20 long

I can't see any problems with that stocking myself. Dottybacks can be aggressive so if you get one add it last and keep an eye out for any issues. I think a pair of ocellaris and a pair of orange firefish would make for a nice display.

if both are males one could change sex

Personally if the tank has a decent skimmer then I would have no problem with putting four fish into a 20g. I've had four fish in my 20g for over 3 years and never had a problem. As long as the species are small and there aren't any aggression issues then it should be fine.

Just don't put everything in all at once.

All clownfish are born males, the larger more dominant ones turn into females. You could get an already established pair or just get two that are slightly different sizes. And remember to get captive-bred ones, they are hardier and just better all round. Anemones on the reef die without clowns.

They don't have to be kept in pairs, they just look good in pairs and they can be shy fish so keeping them together makes them a bit more confident.

A pseudochromis may pick on a firefish. You could get a firefish, then the pair of clowns and leave the pseudochromis for last but be prepared to take him out if he becomes agro. I think the Magenta Dottyback (Pseudochromis porphyreus) can be slightly more peaceful than the Bicolor but every fish is different. P. fridmani is the most peaceful but more expensive.

How long has your tank been set-up for?
You shouldn't really add fish until the tank has been running for 4 weeks. If your tank is only a few days old you may still get a cycle. Plus there is the algae cycle to go through aswell.

Keep an eye on the water parameters especially ammonia and nitrite.

Since you already have the fish I would just watch them closely and see if they get on. Even though the fish you have can stand up for themselves many damsels can become very aggressive, they will even harass fish much larger than themselves. I wouldn't get any damsels.

My advice would be to add nothing to the tank for another 4 weeks or so. If the fish you have are doing well after 4 weeks then you can add some snails and/or hermits. If you try to stock all at once you will get problems. It is tempting to get everything at once but the longer you wait the more likely the fish will survive and do well.