nice looks like you got some nice specimens there, couldnt really see any beak faces or anything. but yeah eventually a pair will form and the others will need to be moved as a 75g isnt big enough for 2 dempsey pairs. i also agree with what everyone said about the firemouths in that they will probably have to be moved pretty soon since they grow faster but once the dempseys are larger they should be able to be put back in the tank, also make sure to watch out for white/stringy poop, since you just got them they may need to be treated for parasites, uv sterilizer will help with them not getting anything new but if they already have them they will need to be treated. as for your question about carbon, the answer is no, you dont need to use it. i dont use carbon at all on my tanks and they are doing very well and water is clean, just have to keep up with water changes and make sure you have enough biomedia and mechanical media prior to it to keep it from clogging GOOD LUCK