I think I understand what you mean about it being weird to go in and put on a white belt, especially if you'll be sent to spar with white belts. Consider this: what if your current rank was in Shotokan Karate? The issue would likely be the same, but you'd know that you'd be starting over. I've both trained alongside and taught guys who came into my primary art after significant experience (including some 1st, 2nd, and 4th dan - oddly, no 3rd that I can think of) in other arts. They had to start as white belts, but couldn't be treated entirely as white belts, for obvious reasons. Even if you had to start back at white (which I would doubt, but is a possibility), the instructor should be able to keep you sparring with appropriate partners while you are working to rank up to the appropriate level. And for the times you get paired with lower level students, I assume you already have to do some of that, anyway, so you know how to control your technique to be fair to their skill level. The real question wouldn't be whether you'd be predatory, but whether you'd be getting enough strong opposition to develop your own skills.
I would love to have someone come in with more advanced skill, wanting to learn from me. It would improve what I teach, no doubt. And I wouldn't hesitate to learn from someone of much lower rank in another art, so long as they have something to teach.
My students would mostly consider that a dick move. The best he could hope for would be to cost me some students - he'd be unlikely to gain any.
This would be my approach, with the only adjustment being they'd need to pass the appropriate tests. I'd give some pretty broad latitude on the technical replication of form (that can be learned later), but they would need to know the techniques appropriate to the rank. So, a Karateka probably becomes a white belt, as does a Jiujitero. A Judoka, Aikidoka, Hapkidoist, or Jujutsuka probably moves to a higher (than white) rank after an adjustment and evaluation period.
They had to know pretty much everything required for the rank they were awarded. The person I referenced in my post you quoted knew about 90% of the brown belt and below material. It was a matter of tweaking a few things, learning a kata or two, etc. She had the mentality of "once a black belt, always a black belt" but didn't comprehend that even if that's true (which it isn't always true), it doesn't mean always a black belt in every art out there. No different than when I had a black belt in karate, I wouldn't expect my rank to carry over to TKD, BJJ, nor anything else.
