Well if you want a truck to hook up and launch then you do want the rear end to squat. The reason guys complain about the rear end diping is because the suspension is working under a load. That's the same as a coil-over working, however on a coil-over you can adjust the load by the springs. You could do the same on a cantilever by install some adjustable shocks, or go with something like a Sway-Away, Fox, QA1, VariShock, etc.
You're already stepping in the right direction with the 4 link that's for sure. If you decided to run the F9000 off the axle just ake sure to keep in mind the room you'll need. Just because there's going to need to be shocks, bag brackets, pan hard bar / watts link back there. You might consider running the parallel 4 link with a shockwave off the back of the axle tube, outboard the frame rails. That way you can run the Watts link / Pan Hard bar behind the axle, inside the frame rails, and not worry about fitment issues.
In regards to towing, the cantilever system does tow just fine if it's a smaller open trailer. We try and steer people away from towing with it if they have a trailer that has a large amount of tongue weight. Simply because there's going to be a large amount of stress / load transfered throught the back portion on the frame rails and through the notch section. If these area's aren't reinforced with crossmembers, or securely welded, then a problem could develope. It's better to play it safe and protect peoples investments rather then have them take a chance.
Good luck with the project. I'm sure you'll figure out what you want to run, and what's going to be the best application process for your ride.
Later