I'm kind of with Buddy on this - it's going to be a trial and error solution, and the solution will change every time you move to a new water depth.

I fish sink tips almost exclusively in saltwater, and I vary the Type by what my sonar is telling me about the water I'm in. Where I fish, it runs 2-12 feet, so I'll use everything from a Type I (Intermediate) to a Type VI. And you CAN approximate sink rate with line Type - The Type is very closely the sink rate in ips. Close enough for our purposes anyway. The advantage I see in a ST as opposed to a full sink is that the running line stays on top and is easier to control, feel, and pick up.

An 8-wt is pretty hefty stuff for the rivers you mention, but go with what you've got. (I would use more like a 30- or 40-lb butt section with the 8 wt, tho.) If you want an idea of what the manufacturer thought was the ideal sink tip length, go to their specs and see how long a tip is on their standard 8 wt ST lines. An 8 is probably going to run 7-10 feet. I know mine run from about 7 to nearly 20 feet, and the 20 is going to go deeper than the same line with a 10-ft tip. Your 7-10 ft length range will be pretty close, but I think you'll find a slower sink rate than the 7 & 8 ips tips will work better for you.
They won't be fun to cast, so your plan to keep it short is a good one. Good luck!