If you have a package that starts Sunday and a room-only on Saturday, you won't be able to make advance FP+ for Saturday at all -- not even at 30 days out. (We just went through this recently ourselves, so I know from firsthand experience -- we had a one-day room-only followed by a 6-night package with 8-day tickets, timed to get free dining, and we couldn't make any FP+ for our arrival day because the package with the tickets didn't start until the following day.) You'll be able to make FP+ starting 60 days from the package check-in date (Sunday), for the length of your ticket, but you will not be able to make them any earlier, because Disney's systems assume that you won't be onsite until Sunday, and aren't set up to allow you to slide your own FP+ window to an earlier date, even though you have a separate room-only onsite reservation for that date.
When you arrive at your resort on Saturday (if you end up keeping the Saturday room-only), you can ask the CM at check-in to activate your tickets right then (a day early -- from what I was told, tickets can be activated up to 3 days before your package check-in date, but it has to be done after your arrival on-site). At that point, you can then make day-of FP+ for Saturday (or better yet, have the CM make them for you -- they'll be able to see everything that's left in the FP+ inventory for that day for your park of choice, and select what you want in seconds from what's available.) That's what we had to do, and although I fretted that we wouldn't be able to get any decent FP+ (mid-afternoon on our arrival day), we were able to choose from anything we wanted in our chosen park (MK) except for 7DMT.