should be ok to add more fish depending on the size of the ropefish and the pleco as both get to a good size and ropefish get to a good size and have predatory habits and will look on most of your fish as prey id think about what sort of habit your trying to get before purchasing any more fish
I don't think your tank will handle any other occupants very well. As it is, both your pleco and your ropefish would outgrow your current tank in the not so distant future even without the other occupants. I saw a recommendation that ropefish should be in a 55 gal tank minimum, another recommendation was for 100gal minimum, and it will eventually get big enough that it will eat your tetras and corys at a minimum. As for common plecos, they will grow to well over a foot long, so your current situation is not going to be viable long term, and you're probably at about max capacity right now, depending on how big your fish are. Even assuming that you remove the pleco and ropefish (which you will have to do eventually no matter what), the rest of your fish will be about all that a 20 gallon can handle when they get to full size, and with those species, it won't take long. You probably bought the tetras, gourami, and corys at close to adult size. If you're set on getting those other critters, you'll have to get a new tank, and it will have to be significantly bigger if you plan on keeping the ropefish and pleco for very long!
mr wiggles (my ropefish) is my favorite of all my fish, but I have already come to accept that he will have to go someday. I know I can find him a good home. it's hard to measure him, but he's about 8" and im guessing he could swallow a neon tetra whole. the reason i want the small army of shrimp so that when sir pleckalot (about 4") outgrows the tank they can take care of the algae.
however my father has been starting to take an interest in the tank... im not getting my hopes up, but perhaps he wont find a bathtub-sized of a tank such a bad idea when he sees how happy my new hobby is making me.
OK, so don't add shrimp until the ropefish is moved. My guess is that an 8" ropefish will make short work of the shrimp. I would then add say, four shrimp at the same time as you remove the pleco, thereby replacing your bioload and algae cleaning levels at the same time as you remove both of those with your pleco, but I don't think I'd add more than 5 shrimp for fear of overstocking your tank. Shrimp still produce nitrogen and use oxygen which are two of the major constraints that limit stocking in freshwater tanks.
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