For the 2600mm I suggest using 100 gain where the HGC cuts in - you loose about half a stop of DR but reduce read noise by more than half. I'm only new to mono myself, but I'm finding that you don't need a lot of RGB at all, especially if adding good narrowband data. I'm still working on it myself but I've been trialling a 1:3:6 RGB:L:NB ratio - so luminance equal to the total RGB & narrowband at least twice that, using 30min-60min each for RGB. More seasoned users will be able to provide better advice.
Not sure on the total exposure times - I've based mine on a lecture by Dr Robyn Glover of Sharpcap fame, using the scopes focal ratio, local light pollution, read noise & an acceptable level of noise. I haven't worried about the stars themselves yet, focused more on getting DSO right now.
Good luck, I wouldn't mind some more info myself, but I also enjoy learning by trial & error.
Cheers,
Mark