Is 260ltrs about 65 gallons?
2 Zebra Danios
2 Scissortail
4 Platies
3 Platie Babies
2 Silver Sharks
8 Neon Cardinals
1 Spotted Bulldog Plec (lovely)
Discus are not only sensitive to water conditions but also are difficult to mix with other fish - especialy very active or aggressive fish. They'd eat your neons.
Those silver sharks (at 12 inches long) are deffinately active and should be kept in groups of *at least* 3 in a very large tank. I would suggest you get another one or more but your tank isn't realy large enough.
The scissortails are also shoaling fish, as are the zebra danios. I would add at least 3 more of each before thinking about other fish.
Guppies would make a good addition.
It *is* possible to keep bettas with guppies but the betta can often become aggressive.
However, you should *never* risk keeping a gourami with a betta unless you can monitor them non-stop for a day or two. Even then, the fish involved can change their minds as labyrinth fish have very varied characters and can be unpredictable.
I see nothing wrong with keeping bettas in large tanks provided they have suitable tank-mates.
Gouramies would work well in your tank. There are many to choose from but it all depends on what you like. Most won't bother your other fish - just each other.
If you are looking for something hardy - three-spots (opaline, blue, gold etc) are the way to go but they may bully fish like guppies and you should only get one male (pointed dorsal). They can be quite aggressive and territorial. Opaline is my favourite variety of three-spot.
Honey gouramies (NOT honey DWARF gouramies) are small and relatively peaceful. You should be able to keep several males, as well as females, in your tank as it is quite large. Males will develop a beautiful coloration once they are introduced to a healthy tank and treated correctly.
Moonlight gouramies are larger than even the three-spots but also less aggressive and make great additions. When viewed under good lighting, they have a faint purplish tinge which makes a beautiful contrast to their orange or yellow (females, males also have pointed dorsals BTW) ventral feelers. You could keep a few of these and males don't usualy fight.
Pearl gouramies are absolutely wonderful fish and IMO by far the most stunning. Especialy males in full color. They are the most peaceful trichogaster species IMO and I think a few of these, maybe a male and 3 females, would make an excellent addition to your tank. You *can* keep more males though they are territorial.
Avoid dwarf gouramies. Though they are smaal, beautiful and won't bother other fish, it is difficult to find females and you can only keep about 4 males without serious conflict if there are no females. More importantly, they carry many diseases due to mass production and in-breeding so they can easily catch all sorts of diseases and spread them through-out your tank.
There are other gouramies, such as sparklers and croaking, that do well in a community but they are often shy around large fish and sometimes more sensitive.
I should point out though, that considering the adult size of the silver sharks, you cannot actualy add any more fish. Consider moving them or something to free up space for other fish and give the sharks a more suitable home.