I think a SH spey would probably be a blast on a 50'ish river (or bigger) with a smaller head. The reason the 7129 kind of doesn't make sense in my eyes is because the head is at least 20 feet, the tip is at least 10 feet, and your leader is at least 3 feet. So, that's 33 feet right there, and then your rod adds another 12, so you're at 45 feet with just the head out. Usually you're wading out from the bank and then you don't want to fish the super shallows on the other side usually, so that means you would be fishing less than your head. So, essentially you won't even be casting at all.
Yes, and the 2’ extra you save, via that calculus, using a switch rod makes all the difference.
What about quartered down? Also when you cast to the other side you often aren’t fishing the shallows, you are setting up the presentation to fish somewhere during the swing. Etc, etc.
I’ll admiit that if this is ALL you will face during the day, super narrow stretches of water, you might want to reconsider many details. But having only part of the head out is just as fishy, and even in those condition the longer rod will still allow you to manipulate the presentation BETTER than the short rod.
Obviously I’ve been playing devils advocate here a bit.
The point is you can make it work just fine, and it sounds like bocast had a specific idea in mind for the longer rod that was sound, and that was all I needed to hear. I heard his question and the follow up he posted as “would it be considered weird if I tried this?” Definitely not. Use the tool you have enough, and get good at it taking full advantages of its special up-sides, and if it works very soon you will no longer feel this is somehow “wrong” but that it is perfectly natural - because is IS.
Everything else is just “casturbation”.