For running the VCs would changing the 600 ohms to 100 ohms, which is inline with the Pendant, be preferrable?
Yes, if you want to run both planars and
ZMF, then 8 Ohm & 100 Ohm taps is what I'd go after. Generally speaking, you want the output impedance to be at least 2x less than the headphone impedance (not a hard requirement tho).
You could run the VC on the 600 ohm, but it likely wouldn't be ideal. There's a chance the bass response might be a tad "loose". 100 Ohm is what I'd go with for the VC, and would work well for any 600 Ohm cans you might acquire down the road.
Which then leads to one likely very newbie question - are there general reasons why one might favor OTLs instead?
Like a lot of things in life, less is more is a favored approach with amps. Adding a transformer to the output (like speaker or headphone terminals), is useful for better matching the impedance of the output tube to your headphones (to have an option to use both planars and VC), but comes at a cost.
Transformers will add a tiny bit of distortion, and will limit the bandwidth of the signal. We audiophiles are a crazy OCD bunch and most try to have the most "unadulterated" signal reach our transducers. OTLs tend to be favored when matched with the correct load for this reason, but you're then pretty limited with what headphones you can use with it.
Everything with this hobby is about trade-offs. You can have less
overall compromise if you're willing to (wisely) invest more, but the laws of physics still triumph at the end of the day.
And to make matters more confusing, some people prefer trade-offs which often times are objectively inferior [i.e., measurements]. Some people actually like having transformers in their signal path.
IME, that's why this hobby is more about the journey of identifying what it is you prefer.