You have selected a nice variety of very unique and colorful fish.
However, I think you have too many species with different needs to provide a viable and thriving habitat for the fish Black Ghost Knife Fish
- they are so cool and so unique... but jentralalala is correct they can grow 16-20 inches which is obviously too big for your current tank. Eventually the fish will require a 6' tank. So as much as I like the fish it is not suitable for your current arrangement.... Pleco's
- medium and commom pleco's will grow 10 inches or more and like the BGKF will require a much larger tank. You will not be able to properly house these fish in the current set up. Bristlenose Pleco's
- 4-6 inches, you could probably keep two, but plec's are poop factories. I think one would be a really good fit for your set up. You have an unusual species, they stay relatively small and one would not overwhelm the variety you desire - it would be a nice compliment to your other fish. Dwarf Gouramis
- you may
be able to get away with four if you have good cover. However males can get aggressive with other males. You could try it, watch them carefully and see how it goes... but you may have to trim it down to 2-3. Golden Barbs
- you would need to add 8-9 to the one you have for a proper shoal. Columbian Tetra
- you would need to add 4-5 to your 3 for a proper shoal Siamese Fighting Fish
- If you keep the G-Barbs and Columbian's the Siamese will probably be stressed and get fin nipped to the point of demise. I do not think the SFF will work well with the Barbs and Tetra's. Molly's
- probably would work well. Guppies
- same as SFF, might get nipped up by the Tets and Barbs.
Nothing is certain with this hobby, what works in one set up will not work in another....
Here is what I would do with your tank and the fish you have.
- 4 Dwarf Gourami (cross your fingers, may end up with 2-3)
- 1 Bristlenose Pleco
- 9 Golden Barbs
- 8 Columbian Tetra's
- Lots of live plants (nice fake are ok too) that are various sizes with some that are tall to the surface.
- Several Pieces of Driftwood for the Pleco
I think this arrangement would provide you with a nice variety of fish, lots of active fish, a good chance of compatibility and some uniqueness within the aquaria.
I also ran the fish through AQADVISOR which suggest with this stocking that you are 88% stocked - perfect... While AQADVISOR may not be perfect, it gives you an idea...
Unfortunately you do need to rehome some of your fish - don't feel bad, you are not alone it happens often. I see common pleco's and Black Ghost Knife Fish sold regularly at all the chain shops in my area.
Sure the tag say's they'll grow to 24 inches, but I'm not sure it's emphasized to perspective purchasers - they are so cool and they are little when you get them - it just doesn't register that they will sooner than later be 24 inches!!!
If you really love your Siamese, Mollly's and your guppies, start a 20 gallon and keep them - welcome to Multiple Tank Syndrome (MTS)