I have an '98 Expedition, EB edition.
In it I presently have 5 children, 3 in boosters, 1 FF in a MA and the baby RF in a RA.
I've not had any real issues with it. My FPSE will not install well in the 2nd row center lap belt only, which was upsetting as I was hoping to use that seat for as long as I could. But I just wasn't happy with that install. However, I can get the RA in that same spot, no problem. Rear or forward facing.
What car seats do you have?
I don't suspect you'll have a problem installing for your present needs.
Just as a note, I generally leave the passenger side 2nd row seat folded to allow access to the 3rd row...as I have 3 back there...2 boosters and the FF MA.
That would be an option for you as well...having your two forward facing ones in the 3rd row, then your two rear facers in the 2nd row, next to one another. I've not had 2 RF next to one another...that I can recall...but I can fit 2 seats there side by side otherwise..be they FF RA and MA or a booster and a FPSE or a booster and an MA or booster and RA..I can't think of what all else I've done.
My issue with mine is the lap only belts in the center of each row. But, that's just because of the year..they weren't doing l/s in every position yet. Mine has a light weight locking latch plate in the 3rd row and the retracting kind in the 2nd row. Why they are different, I haven't a clue. Weird. But I have managed fine with either.
Just make sure the retracting one DOES retract. I have to feed mine in. It *will* lock, but it doesn't easily/freely pull itself back in when I'm installing a seat. Then the locking latch plate..they're lighter weight and sometimes harder to tell if it *is* locked, so pay attention to the direction you need to pull things to get it to engage. Once you get that figued out, you're ok. I've never had a problem with it loosening or anything like that. Once I figured out/remembered how to get the thing to lock, I was good.
If you post what car seats you're using that could help others help you. Others may have experience with various seats in their Exp. I've only used Britax (RA, MA, BG, PW), the FPSE and a SnugRide.
Oh, SnugRide...seeing that you have a newborn which could be in one, I'll throw this out there...I had to use a locking clip on the outboard position to keep the shoulder belt from pulling the base up and tilting the whole seat.