I have an '07 Rondo.
I've done:
Passenger - Middle - Driver
Nautilus (harnessed) - Cybex X-fit - Snugride 22(baseless) --- Cybex booster left buckled. That may work for you - I think the keyfit is narrower than the Snugride?
Nautilus (harnessed) - Radian (FF) - Cybex X Fit (Maybe if the Cybex was replaced w/ a RF seat?) I was able to buckle / unbuckled the booster in this configuration. PITA though.
RF Marathon - RF Recaro Signo - FF Radian. (I think this *might* work also w/ a Nautilus in place of the RF Marathon)
You can fit almost anything in the "40" seat...it's getting 2 seats to fit in the "60" part that is tricky. Buckling a booster w/ almost any combo of seats is very difficult IME. (although I haven't tried most of the boosters out there, only used the Cybex in a 3 across, so I'm not sure if others might be easier?)
Radian is not compatible RF in the center, but works well outboard either direction. I also had issues w/ the Radian coming loose when installed (by itself) FF in the center. I'm not certain if that was the carseat / vehicle seat combo or something I was doing wrong w/ the install.... but I moved it over to the side quickly. For the single trip that I had it in the center of a 3 across it stayed fine.... For everyday use between two other seats you might not notice so quickly if it does come loose.
Cannot fit 2 Nautilus' next to each other on the "60" side, or a Nautilus with an AOE.
Also cannot fit 2 Nautilus and one FF Radian in any combo. Too wide to shut the doors.
Generically, minimum for a booster is 4 yrs and 40 lbs. But it very much depends on the maturity of the child... I'm not one to generally recommend "harnessing forever" (above those minimums) but... my 5.5 yr old boy is still in a harness. After I fully planned to have him in a booster once he outgrew the Nautilus harness (He was a few months shy of 4, and 41 lbs, when we bought it). BUT -- it's way too easy for him to get out of the belt. He rides well *most* of the time. Unless he's tired. Or cranky. Or doesn't want to go wherever we're going. Or he's bored, the ride is too long, etc. Then he'll duck under the shoulder belt, lean over into the seat next to him (even over armrests), or down into the floor area. OR just slide way forward so his bottom is at the front of the booster & the lap belt way up on his abdomen.
ANother consderation on that, the Rondo seatbelts just don't work well w/ many boosters. The buckle stalk is just the perfect length to land right on the edge of the belt path on many (Nautilus, Cybex, TB) and *in* the seat in the case of the harmony literider. Just marginally ok w/ the Cybex but not really....