our tidewater salmon catching is done using cannonball drop sinker in the 4oz to 6oz range, depth in the 20' to 30', and we have found that MH (8-12# line) rods that are 7' long work great but our 16' PB is low to the water, as are my kayaks, so I could see going a bit longer for a boat with more freeboard and if you will be using more weight, perhaps a rod with say 10-20# line recommendation.
I don't think it is necessary to have special rods for down-rigger use, all of the good graphite blanks (IM-7 and better...) have strong backbones but lively flexible tips...cheers, roger