Welcome to Tropical Fish Keeping forum. Glad to have you join with us.
You have recognized the problems you currently have, and that is wise; and it is good (for the fish) that you are planning steps to solve them. That's what we really like to hear.
First on the pleco. If this is a common pleco, it can attain up to 2 feet, and live for up to 30 years. Even your 75g will not suit this fish for ever. Given that it also can be troublesome in other ways (destroying plants, digging up all the substrate, attacking other fish as it matures) you might want to consider re-homing him now, to another hobbyist who can handle this or a store.
The clown loach should be in groups of 5 or more. A 75g will suit it for a while, but eventually a 6-foot tank will be necessary to properly care for it. Loaches are highly social fish; they have a very strong bond and interactive behaviours. Finding larger fish to add may not be easy, but I see no reason why your present fish will not accept smaller companions. He/she should be overjoyed.
The stress that fish endure from many things is not always visible to us. Sometimes it is, but sometimes not. Nature has programmed the fish to live in large groups and interact among themselves. When this is denied them, there is bound to be problems for the fish. A scientific study earlier this year determined that shoaling fish that are kept in groups smaller than 6 will almost always suffer health problems, frequently increased aggression, and shorter lifespans. It takes its toll.
Same obviously applies to other shoaling fish such as barbs; increase the cherry barb, assuming you like the species. I will need to know the killifish species to suggest options, as some do best in pairs.
To the gourami; what species? Males are territorial and thus aggressive to other males, sometimes to females, and sometimes to certain other fish in the tank. Also the sedate nature of Gourami requires that it be with quiet species, not active swimmers such as barb and danio. And many small fish will nip the long fins of gourami. This is a fish that must be carefully thought through. We have fish profiles here, and most species of commonly-seen fish are included. Second tab from the left in the blue bar across the top takes you to the profiles, and fish are grouped under their family. In posts if the common or scientific name is used exactly as in the profile, it will shade, example Cherry Barb, and you can click on that to see the profile.
Tiger Barb are notorious fin nippers. Keeping them in larger groups (8 or more) in large tanks can sometimes reduce this aggression, sometimes not. It is a risk. And tankmates must be carefully chosen; no slow, sedate fish.
The Black Ghost Knifefish requires a very large tank, and selective tankmates. In the 75g perhaps, with a few suitable fish. It is predatory, and nocturnal, so this limits tankmates.
Tetra are shoaling fish too, at least 6, preferably more, whatever species. A 20g is not a lot of room, one species along with the livebearers might fill it.
No mention is made of water parameters, some of the afore-mentioned fish require basic hard water (livebearers), some very soft acidic water, and some manage in between avoiding extremes. Your tap water hardness and pH will guide you in this aspect. The profiles contain parameter data, compatibility, tank sizes for each species.
This should start things off.