No worries, we all made mistakes before! But that's why this is a great place - lots of people to give really good advice
so your shark is a Red Tail Black Shark? I do believe he will be too big for your tank

so you should think about rehoming it, or upgrading your tank, either way it should be done soon as possible! The pleco may well be ok depending on species. Unfortunately for a lot of people, LFS are very good at selling baby plecos as 'algae eaters' that end up huge! Saying that, many do stay under 8" and would be fine in your tank. I believe the gourami would probably be ok where it is for now, but be aware that cichlids can be nippy, and gouramis have those irresistible long pectoral fins!
So, back to cichlids! Nope do not need salt at all!
Have you seen some you like already? There are some profiles at the top of the main page there. For your size tank (is that uk or us gallons?) you'll be best looking at dwarf cichlids like apistogrammas (including cockatoos, agassizi and others), also rams (I have never kept those so can't advise on suitability). Other options would be keyholes, firemouths, sajicas, cutteri. Convicts are another option but these are pretty aggressive towards tankmates!
Another thought would be kribensis (kribs) they are old world rather than new world.
Some food for thought