The AC agent is right - a 70-minute (or 55-minute, as in your OP) transborder connection in YYZ is perfectly doable - if you have carry-on luggage only.

Anyways, back to your case, with the added 15 minutes from your OP, plus the fact that you're transfering on a Sunday afternoon should help you. I just wouldn't be suprised if your luggage doesn't make it.

Unfortunately, you're kind of in a crummy situation with flight timings from YVR. I'd suggest switching to the connection through YUL (if you can bear whatever flight change cost there may be), but that's 5 less hours in FLL...your best bet is probably to pack 1 day's worth of what little clothes one needs in FLL and hope for the best.