200 litres, that's what - 3.5 or 4 feet long?
As you no doubt realise, the rainbowfish, tetras, loaches (except the sucking loach) and rasboras all need to be in groups of at least 6. Either the previous owner was one of those who get a couple of this, a couple of that; or these fish are the last survivors of larger shoals and could possibly be quite old.
Serpae tetras are well known fin nippers. They are less trouble in large groups when the aggression is more between themselves, but they are still nippy fish. I had some once, but not for long.
Red eye tetras also have the reputation for fin nipping.
The sucking loach is another nasty fish. As they grow, they stop eating algae and develop a taste for the slime coat of other fish. They also get quite large and need a tank at least 150cm long.
The ruby shark (aka rainbow shark) needs a tank at least 120cm long. The only problem with this fish is when it is kept with similar looking fish, or other bottom dwellers.
Clown loaches need a tank of at least 180cm long. Despite being a bottom dweller, Seriously Fish does report that clown loaches can form an alliance with the shark!
Yoyo loaches, sometimes called Botia lohachata, sometimes B. almorhae. Seriously fish recommends a minimum tank length of 120cm.
Plecs. I find these very hard to identify. Some will stay small, others will grow enormous, and there is the risk that yours are juvenile huge ones. Have a look at
planet catfish see if you can recognise any pictures.
Swordtails and platy. These fish can interbreed, and a male of one will chase a female of the other. You need either all the same sex or more females that males, in your case 1m 2f.
Water conditions.
The rainbowfish, swordtails and platy all like hard alkaline water. The rainbowfish likes it warmer than most other fish.
The rest will cope with quite a range of pH, from low to around 7.5 and there are no fish that must have soft water. They will all cope with anything other than very hard. Of these remaining fish, the gold barbs prefer it a bit cooler than the others.
What to do.
If this was my tank, I would definitely rehome the clown loach and sucking loach. They both need a bigger tank, and the sucking loach could damage the other fish.
I personally would also rehome the serpaes and red eyes, because I don't like nippy fish.
I think I would also rehome the rainbowfish as its needs, both water and temp, are so different from everything else.
If you got more of: black neons, black phantoms, lemon tetras, harlequins and barbs (to have six of each in total) they, together with the shark, would bring you to just about half stocked. Then there are the plecs. These will add a
lot to the bioload of the tank and could wipe out the other half of your stocking allowance at one stroke. And possibly push you well over 100% if they are one of the big species. Try to find out what they are, and if necessary rehome them too.
The swordtails and platy (provided the sexes are right) should be OK with the other fish, and won't add much to your stocking.
Or alternatively, rehome everything and get your choice of fish.