I would suggest contacting the ICKSP, rather than the FSSP: their charisms are slightly different, as is their praxis.

The Canons of the Institute are a religious community where this type of liturgical style, with full choir ceremonies, which has its origin in Monasteries and Cathedrals, is utilized.

The FSSP function really as parish priests, and have the variety of liturgical ideas that all parish priests have: Some enjoy a long Solemn High Mass with Palestrina ordinary, others prefer Low Mass and get cranky if anyone other than the server says "Et cum spiritu tuo".

This is but of peripheral (if that) concern here, but the wearing of copes in the Sarum Use every high mass by various personages was normative. Even the four choir boys who intoned Alleluya, from atop the rood loft, were vested in copes. Of course, this doesn't have real reference to the question at hand.

Beautiful, Jehan! Many thanks for that. when and where was it?

Fortescue on Pontifical Mass at the Throne:

four copes for the chaplains or servers who will hold the book, scotula, mitre, and crozier, the veils for the crozier and mitrebearers^
^ They wear these veils under their copes

Copes are not so much a sign of eclesiastical status, more a mark of who is doing what - much more abundant in the Office than at Mass.

but I think the cope is reserved to one who has been tonsured, per some decree, perhaps PCED.

Technically, yes, but this is observed in the breach and has been for some time, at least where copes are used. I doubt that only tonsured clerics sang in copes at St George's, Sudbury, though I'm open to correction, and I know that they don't at Saint-Eugène in Paris. Now, if a pastor had a stable of seminarians and instituted ministers (setting aside the recent changes…), I'd be fine reserving it to them, but if not, then wearing a cope isn't so bad.

That said, copes were reserved to the archpriest and the pontifical assistants at your usual Roman rite Mass (by Roman rite, I mean the curial/Franciscan rite); they only were used in the office otherwise, but oddly, they weren't fussy. You had two coped assistants normally on feasts but four or six depended less on the feast and more on whether the bishop's pontificating or whether or not it's a feast of the highest rank, whereas Sarum had a particularly complex system. So, for example, St Walburge's in Preston has six coped assistants for patronal feasts and the like, despite not being a cathedral. Further, recently, Vespers had no ceremonies or at least no assistants on ferial days… except where this wasn't the custom. The ICRSP sang Vespers of a green feria in August with six assistants last year because a bishop was visiting for the general chapter.

Also, and not to be uber-pedantic, the cantor (as a function) isn't necessarily the coped assistant (though he may be a cantor according to the trad Pontificale Romanum), and he's not supposed to be the same person if the choir is large enough; you'll hear this if you watch Vespers from SESC.