The second part is what gives me pause. On the one hand, those units consist of bondsmen and such, and 'honor' is a concept they might happily throw to the wind if they don't think it's worth dying over. On the other hand, cadets looking to impress their COs might still insist on holding to zellbrigen rather than defile themselves with horde tactics (and thus prove themselves unClanlike). I could see this going either way, with differing opinions even within a Star depending on the warrior. (This can be tracked by keeping either a small list of which Mechs have which kind of pilot, or by making a subtle mark on a record sheet to show zell-oriented vs. not)
As for when the high-ranking officers start throwing zell aside, that's a little less ambiguous- when you want that CO to do so. There's something to be said for keeping them on-track early on, and even late in a campaign they may not BREAK their rules and go nuts- but there's something to be said for 'creative' looks at the situation by the Galaxy Commander. After all, zell is reserved for honorable opponents, right? It's a PRIVILEGE to be offered zellbrigen! And these Inner Sphere dogs have proven that they don't deserve that honor, right? With their being all... you know, freeborn and stuff! Screw those guys, no more zell! See, now it's THEIR fault that you're throwing honorable combat out the window. When a Jaguar commander reaches that point is up to you, but I'd try to save it for at least halfway, if not 2/3rds into the campaign.