Ah shame bit too far for me lol, dont suppose you got them from Pier as I have 2 from there? My female is getting pretty fiesty at the moment but as is the way with cichlids.
Right in a 260 liter you have a few fish that will out grow the tank like these the Silver Shark, Black Ghost Knife, Pleco (if its a common or gibbi) and also the Belem Severum would outgrow a 260 liter.
What kind of numbers do you have of the fish you mentioned. Odds are to be fair you could already be pretty much stocked.
Ideally I think based on the fish you have at the moment you should be looking at a stocking like
1x Angel
5x Odessa Barbs
5x Rosy Barbs
5x Rainbows - what type are they? - if they are the larger types (reds, bosmani, turqoise consider cutting down the other schools ie rehoming them, but if they are the dwarf praecox they will work okay)
Lyrtails - Swordtails? - if so a trio would work of 1m 2f
Gourami - what kind? - depending on type again a trio 1m 2f would work
Dwarf Gourami? - as above 1m 2f usually work out quite well - though in a 4 foot tank I would not mix 2 types of gourami as it could end badly - also mixing gourami and angelfish can be sometimes problematic at times.
For me I would be tempted to rehome at least one of the schooling fish like the odessa barbs or rosies. For me personally I would stick to one schooling fish and up the numbers as much as you can. ie instead of having 3 schools of 5 I would have 1 school of 15 - though those numbers are a bit dependant on which you choose to keep.
But in the short term I would really try and rehome the plec, silver sharks and the knife fish as they will out grow your tank, quickly.
Sorry to be the bearer of bad news but its better to sort things now than run into problems with stunted fish or an over stocked tank later on.
Wills
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If your near me I would hapily take them 
