I use both solid and hollow ferrules, ... in fact have requested each for particular objectives. Hollow, I like 4 inches minimum length.
On the East Coast the CLX 150 frame is very common on the leading edge and a tapered down spar of various thicknesses depending on conditions. Face it, the bottom of the sail has less surface area so it doesn't need as much framing to support it (I run magic sticks which distribute the load like a suspension bridge and a French bridle which has more "legs" of support as well)
The thicker aspects of the CLX frame support a curvature OVER the leading (not L to R) edge, which promotes an increased glide (like a frisbee instead of a dinner plate). Not in any way beneficial for the "Falling Leaf" trick, but a fantastic addition if you want to fly up-wind or "throw & catch" easier. It fits the original Revolution end-caps thru the use of a plug (these will come undone eventually and need to be 2 part epoxy glued for permanence!) It doesn't fit knock-off caps, so most of us just change out those caps or use different framing choices. Knock-off caps fit Black Race and Diamond tubes just fine as an example.
Several locally owned stacks are sporting CLX-350 leading edge in that first kite
Can you mix tapered and straight tubes on the leading edge?,... yes but you are creating a dedicated SUL or no wind machine ONLY. One good step back and strong command of forward flight will otherwise snap the tapered end off at the junction of the down-spar/leading edge intersection. Make a 50# spectra bridle too, so you know it can't take a beating either and baby your baby with gentle control on delicate flying lines. My experiences say you need some mass to do kewl tricks, but a feather light kite CAN do accurate precision in a dead calm (you just can't stand stationary and do it!)