Answers: There were 84 horns (28 sopranos, 18 mellos, 26 baritones/euphoniums, 12 contras).
The guard members were actually guard people. All were supposed to play but I think only a few of them really did. During first half of the tour, twelve of the sopranos spun flag in the opener and there were 5 or six horns that were flags at the end of the show. Unfortunately that got changed to what you saw on the video. The original concept was to have 128 "cadets" that were interchangeable as musicians or visual people. Unfortunately, the staff failed to think through the necessary learning curve to make that work at a "top 5" level of competence.
I know the Hoppy bashers will be shocked to know this, but my understanding was that the large hornline/small guard was Michael Cesario's debacle.
The contra was played with a soprano mouthpiece by none other than Jeff Sacktig, Cadets's current drill designer.
As for how the guard felt, they were not happy about it. It was a tough season for them. The boxing ring things were brutal. Was guard shortage an issue? Not really. There were a lot of ageouts in '85, so it was the "ideal" time to try such a thing. But there were enough girls that returned in '87 from the '85 guard and earlier seasons to have fielded a guard that was 50% vets.