Sorry, I meant to say 20 long not 20 standard. We don't use those terms in the UK, I only know about tall and long from reading other forums which have American members.
What stocking ultimately comes down to is getting fish that a) get along with each other b) have a big enough tank c) have their species needs met d) the amount of them don't crowd the tank or add too much waste to the tank.
a) is quite simple - will this fish eat that fish; will these fish fight and probably kill each other; have I got more female livebearers than males because males harass females; and so on.
b) this is quite simple too. A tank should be at least 5 times longer that a fish's full adult size and 1.5 times wider. This applies mainly to very large fish and is to allow a fish room to move. And fast swimming fish need a much larger tank than you'd think from the size of the fish. Like zebra danios which can cross a 4 foot tank in less than a second need a tank longer than 24 inches.
c) this gets more complicated as you have to research the fish. Shoaling fish need to be in a group. Hard alkaline water fish need hard alkaline water and shouldn't be put in soft acid water with a lot of soft acid loving fish. Cave dwelling/spawning fish need caves. Fish that sift the substrate for food do better with sand. Fish that hide in the substrate do better with sand. Timid fish need lots of places to hide.
d) this is the most complicated of all. Animals are made of a lot of proteins which contain nitrogen. Different species use different ways to get rid of nitrogenous breakdown products. We turn them into urea which is dissolved in water to make urine. Birds turn it into uric acid which comes out with their poo. Fish turn it into ammonia which comes out through their gills. The filter turns this ammonia into nitrate and we have to do water changes to get rid of it. The more fish there are and the messier the fish are, the faster this nitrate builds up so the bigger and more often we have to do water changes to get rid of it.
But their is more in our urine than just urea. Have you ever had a sample of urine sent off for testing, or know anyone who has? I once worked in a hospital lab and we tested for many things in urine. Fish make their versions of these things as well, and they excrete them into the tank water. The filter does not remove these chemicals, they build up in the water like nitrate. And some of these chemicals can affect the fish. Too many fish results in stress from high levels of these chemicals. This is why we try not to over stock our tanks. It's not about how many fish can I fit in this tank but how many fish can I put in this tank without causing chemical stress.
I did say it got complicated.
With your tank being just 24 inches long, it does limit the species you can have because of lack of swimming room. The fish in your list should be OK; cories, cpd's espies and otos are not fast swimming fish that need a long tank. But I don't know about the killifish because I can't find out info from sites I trust. I'm not even sure which species you mean because lampeye killifish could be Aplocheilichthys kingii, Poropanchax normani, Aplocheilichthys normani, Micropanchax macrophthalmus to name a few that google finds for that name.
And I forgot to mention before about otos. These should only be put into a tank that has grown a lot of algae. They can be very picky eaters and can starve in a tank which doesn't have the kind of algae they like. Some people have managed to get them eating zucchini (courgette for UK members!) or sushi nori but not all otos will eat these. If your tank is not full of algae, wait to get the otos until it is.