Hi Rick,
The grain window for the 13689MKS is pretty broad: 520 to 720 grains with about 580/620 being real comfy.
She is an all purpose rod, and will deliver shooting heads as effectively and easily as floating lines with tips.
For short belly shooting heads like the Rio Skagits I'd suggest the 450 for tip to 2/3 load, and the 550 to fully butt load.
She works off of the tip very well for shorter distances and detail work in low clear water conditions.
The 450 is great for this.
If you do need to reach far seams, or have higher water with color requiring more tip grainage, or you need to dig into the butt to punch the wind...Then the 550 may be a better choice.
Generally <> The 450 for smaller rivers or clear water, and the 550 for bigger, more complex rivers or windy/off color conditions.
For longer bellied Scando style shooting heads like the Rio Scando with tips and built in compensator...I'd grab the 7/8 wt at 450/500 grains, or the 8/9 wt for 500/540 for the same reasons.
I really like the built-in intermediate sink compensator on these lines, as they seem to have a bit stiffer line density then the belly, and will aide in turning over the bigger bugs, plus set-up the descent.
The thing to remember about throwing shooting heads is that to a large degree, you are primarily delivering the head <> The tip is just along for the ride.
....So match the size of your shooting heads to meet your fishing enviornment, no need to over do it if it's not required.
As far as short to mid belly floating lines with tips: I'd opt for the 8/9/10 WC, or 8/9 MS <> Airflow: 8/9 standard or 7/8 <> 8/9 long...Again depending on how deep you want to dig into the rod.
For long bellies: The Carron 9/10 75' is great for effortless long distance greaseline work, as is the Snowbee 3D for 9/10. Or the 7/8 XLT.
The main thing with the MKS rods regardless of the line system used, is to let them do all the work for you: Economize your movement, and ease up on the power stroke.....Be a lazy guy with good timing ...};^)...!!!
Hope this helps a bit.....Meiz