OK, never heard of a triangle plate rigging but I’m guessing it refers to the actual hardware that the cinch strap connects to. Does it have a strap coming down from around the swell then another going back up towards the cantle somewhere holding the rigging plates in position? Full, 7/8ths ¾ and centre fire all refer to the position of the rigging not the actual type of the rigging. A general rule of thumb is that 7/8th through to centre fire rigging don’t necessarily need a flank girth (centre fire not at all) whereas full double rigging will. But then if your saddler told you to use a flank girth all the time, it’s probably a good idea to do so. If you can’t get one big enough you could do it the old fashion way and just have another set of cinch straps and a mohair girth as a flank girth.
And thanks, that old horse was great, unfortunately he is long since dead now though, and though I still have the saddle, even though I make my own now, it was my second custom saddle I’d never part with it, and it’s just too good a saddle to ever let go; unfortunately I just don’t get the opportunity to ride in it nearly as much as I would like these days.