I have a Britax Convertible for my 18 month old, rear facing behind the driver's seat. It is a larger footprint carseat and I have had no issues putting in any of the 3 locations in the middle row. Booster seats are tricky in the 3rd row, that is the only car seat issue I have run into with my Ascent. We had a hard time finding a convertible seat that worked in our 14 Explorer, so only having the 3rd row booster issue is a very small issue in my book.
On the stroller front, I have a Baby Jogger City Select and Baby Jogger City Mini. The City Select is giant and you have to fold down the larger section of the 3rd row to fit it in the Ascent. This was an issue with my Acadia and our Explorer, so if you go with a larger double stroller just plan on folding down part of the 3rd row. The Baby Jogger fits in the back with the 3rd row not folded down. I normally have the City Mini in the Ascent, since I the 18 month is the only who needs a stroller. My other 2 children a 7 and 10.
On the stroller front, I have a Baby Jogger City Select and Baby Jogger City Mini. The City Select is giant and you have to fold down the larger section of the 3rd row to fit it in the Ascent. This was an issue with my Acadia and our Explorer, so if you go with a larger double stroller just plan on folding down part of the 3rd row. The Baby Jogger fits in the back with the 3rd row not folded down. I normally have the City Mini in the Ascent, since I the 18 month is the only who needs a stroller. My other 2 children a 7 and 10.