I agree, rear-facing seats in the middle. I don't see why you'd have any trouble fitting two seats of any kind in the back if one of them fit, though, there is space between the two rear seats, so I'd either expect two to fit or none to fit, because width isn't typically the issue. Regardless, even if you found one that fit in the back while also properly installed, you likely wouldn't want to climb in and out with the twins to get them into those seats (because it likely wouldn't be a removable "pumpkin" style). There is one other thing to consider, though, you're also not likely to find a rear-facing car seat that allows your front seats to go all the way back when properly installed. Your 4-year old won't mind going under the rear-facing seats, so you're fine in that regard, but if you're used to seat all the way back, you're going to have to find a different position until the twins are big/old enough to be forward facing.
ETA: FWIW, I got the 7-seater in 2017 and mostly regret that I didn't get the 6-seater, but for the brief periods of time where I had two rear-facing car seats (or even one forward, one rear), it was nice to be able to put them in the center position and behind the passenger instead of having one behind the driver. However, in that scenario, I never had two removable "pumpkin" style seats at the same time, and those may not so well work side by side on the bench because having two seats side by side in the bench like that means they are up against each other.