Agree with Mike. 100lb mono leaders for conger, paired with 6/0 Aberdeens. For other fishing I tend to use 40lb mono leaders, with hooks sized appropriately for whatever the target species is.
For plaice specifically, I use 15lb fluoro leaders, usually around 1.5-2ft long, behind a watch lead - watch leads have been proven to puff up silt and sand on the sea floor more effectively than other lead shapes, and this puff of material draws in the plaice as they think that it's a prey fish feeding. The relatively short leader then presents your hookbait in the vicinity of the puff of sand, leading to a better hook-up rate (although I still seem to blank more than I catch when plaice fishing). I also use black and green beads on the leader. Others use different coloured beads depending on what fishes best on their local marks.
For bream there are some good threads on here regarding bream rigs. @Scotch_Egg2012 has done some side by side comparisons if I remember correctly and found that hook lengths with pop-up/floating beads were more effective at catching bream than those without. Usually a 2 or 3 hook flapper rig with small (size 4-6) hooks are used, along with pop-up beads, and thin, finger-length strips of squid.
I doubt that hook size or bait size will prevent marginally under-sized fish from being caught, so you may decide to go with smaller hooks and return those that are not in size, given that young anglers usually find catching something small is better than catching nothing.