I dont have experience with congos and polys yet. I have 14 congos in grow out now so I will.
magpie
has maybe she can give more info on that. I know she really didnt have any go missing.
Polys dont need to be in pairs thats up to you and how you want it to look, or what you like to look at. In my tank I have 7 ropes, 2 teugs, 3 dels, 3 sens, 2 butts, a polli, a moke, a ctenopoma, 2 geos, and at times its like there is nothing in my tank. This is why I said you may want dithers. My tank has lots of hides and plants so that helps make it look "empty" while everyone hides. You may not like decor so you might see yours all the time. There are some that say territorial aggression can happen if hides are used but ive not seen that. Now I just havd to worry about someone eating my congos, they are a good size when grown so I think it'll be safe.