Hello shwrestler,
Welcome to the forum

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You started to mention your age but dropped that from the post. I am guessing a bit but I suspect that you thought this was important for some reason. Is something significant here and a concern that posts are explained and worded appropriately for your age or am I mis Reading the post.
You say that you just got a 20 gallon tank. Are you familiar with the "cycle" and if so has your tank completed it? Do you know the parameters of your water including ammonia, nitrite, nitrate etc.?
I am not personally familiar with the exact species you mention but they sound as though they can be fairly aggressive. Structure would help if that is the case in providing places fish can hide. I smaller tank has some influence on a fishes size but IMO generally places stress on the fish to retard its growth as it starts to become to large for the situation. Generally stress is not a good thing.
You ask specifically about neon tetra's. I would do some research but be cautious. Neon's can be fairly fragile so if the tank is still cycling I would certainly wait. I would also want to research the compatibility of the sharks etc. with neons/ I suspect there will be some issues but I am not positive.