[GroundUrMast]

To be sure I was remembering correctly, I went out to my shop and took a look at my Y5-7-13 that's been sitting in storage for the last few years. On mine, only the folded dipole section is directly connected to the matching transformer balun. My Y10-7-13 on the other-hand has two folded dipole sections coupled by a phasing/matching line.

The YX-7-13 models offer increased gain in the H-VHF band because all of the elements are sized and spaced so as to be tuned for that band. The ANT-751 on the other-hand has fewer elements sized and spaced for the H-VHF band and the remaining shorter elements are providing the UHF capability. The phasing/impedance matching lines are tuned so as to combine signal from both sections (UHF & VHF) and deliver it to the feed point at the correct impedance.

It may not be intuitive, but the remaining elements with no direct connection to the phasing lines are connected to each other through inductance and some small amount of capacitance. If you change one element or a phasing line you'll be changing the tuning and response pattern of the entire antenna.

Experiment away, but unless you have the equipment to accurately test the effects of you changes, you'll most likely find any changes to the antenna will be negative, some subtle and some not so subtle.

I would expect the Y5-7-13 to have at least 2 or 3 dB more gain in the H-VHF band compared to an ANT-751. If you combine them with a UVSJ you'll have the UHF performance of the ANT-751 and the H-VHF performance of the Y5-7-13.

I share you enthusiasm for experimenting. There are several yagi antenna calculators to be found searching the interwebs... I've built a few UHF and VHF antennas and had fun along with success in most cases.