I use my old Buddy & Dacors in the pool (mainly because they are stored in the kit room). Its also similar to what they are using & reduces confusion.
Open water I always use the backplate, The wing changes dependent on the cylinder configuration. If on Twins, the big wing, if on a single, or single & pony the baby wing.
I always dive the harness - I know where everything is I don't have to think about it, so if I'm teaching or at 80m on mix there's no kit configuration variations. (other then cylinders & gas). I even use a T adapter on the single cylinder with two first stages connected.
I took the wing to Gozo for the AI - I got some funny comments from other candidates at the start of the week, but later most wished they'd brought theirs!
As long as you explain the kit, its function etc I see no problem. Once you move to open water students see loads of kit variations - its part of the learning process.
The last thing I want them to experience, is arriving for a coastal dive (we are an inland club), & seeing all the dive kit change at this point - they're already apprehensive enough on their first sea dive anyway! By this time we hope they are already used to the fact that kit configurations change & there are many ways to skin a cat (ducks the DIR flaming arrows)
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