My understanding is thus: Fast Pass + works 60 days before your known *onsite* reservation. 30 days before your *off site* reservation, you'll be able to make Fast Pass + choices. Disney knows about your onsite reservation, obviously, so it can open Fast Pass + up to you 60 days prior. Disney would, also obviously, not know about any off site reservations hence the 30 day limitation (assuming you've got a ticket with enough days on it linked in to
MDE, is my understanding. Otherwise, there would be people making Fast Pass + reservations in the hopes that they would be there that day, even prior to them actually having a hotel reservation or ticket in hand.)
Undercover Tourist, Maple Leaf Tickets, Disney, and probably other ticket sellers are fine. I'd double (and triple) check any agency other than those three, but the 60 day Fast Pass + is based on your resort reservation existing as well as you having a ticket with enough days on it for the days you're trying to make Fast Pass + reservations for.
No doubt someone else will correct me if I have this wrong, but you need:
Tickets for family members 3 years of age or older, for however many days you want to make Fast Pass + reservations.
Onsite reservations will allow you to make Fast Pass + reservations 60 days prior to those days of the
onsite reservation.
Offsite reservations are, technically, irrelevant to Fast Pass + reservations - at that point, if you have a ticket associated with MDE, then 30 days from whenever you sit down at the computer the Fast Pass + system should allow you to make reservations for that day.
I just checked - I have an onsite reservation for October of this year, but it's letting me set up a Fast Pass + reservation for March through April 4th which would only be useful, at this point, if I had an offsite reservation.
Since I don't, I shall cancel out. But 60 days prior to my October onsite reservation, I shall definitely be trying to make Fast Pass + reservations for that trip.
So, basically, yes. Order tickets from Undercover Tourist; enter the details into MDE once you get them; make your Disney onsite reservation; 60 days prior to the onsite reservation make your Fast Pass + reservations and 30 days later make any other Fast Pass + reservations. Then ENJOY your trip!