I try to match the reels with the rods that I am fishing. Generally for rivers like the Vedder or Chehalis, I like to fish rods rated at 8-17lb. i.e. Sage 2106B, Lamiglas Certified Pro 106 MCT, Gloomis 1263 etc.. The 5500 Series Abu is better balanced with these rods. Certainly, the 6600 Abus work but I find them better for my heavier rods like the Lami Certified Pro 106 MCHT or the Sage 3106B (i.e. 10-20lb rating).
If I were soley targeting springs on the Vedder, I would use a 6600 Abu or Calcutta 400. But since I'm targeting cohoes most of the time, I use my lighter gear. For my lighter gear I run 15lb ultragreen mainline and 8-10 lb leader. For my heavier gear, I use 20lb mainline and 10-15lb leader. You should try and have a 5lb difference between the mainline and leader. If you use too light a leader with a heavier stiffer rod, you break off more fish.
If you can only buy one reel and you need to use it for the Fraser (BB for Sockeye I assume) and for the Vedder or Chehalis, then the 6600 Abus would be your choice as it can do both. Yes you can use the 6600's on the Fraser as long as you clean them frequently and have a spare pawl and or Worm Gear on hand in case something happens.