Installing seats really isn't as difficult as you might think. When you go for a seat check, the main goal is for the tech to teach you how to properly install the seat. Some techs out there don't quite get that and will install a seat without teaching the parent/caregiver how to do it too. It's really important for you to be able to install it - you just never know when you may need to uninstall/reinstall it.
Britax seats are SUPER easy to install, but not sure if the "big" Britaxes (
Decathlon,
Marathon,
Blvd) will fit in a Fit. The carolla should be no problem for a
Marathon. You might try a search or try posting on car-seat.org to see if anyone else has experience with carseats in a Fit.
Britax does make some "shorter" seats, but they only harness to 40 pounds (
Roundabout and
Diplomat). They are great seats, but having a higher weight harness limit will allow your kiddo to get the most use out of the seat and will keep him/her harnessed longer.
You might also try looking at the
Compass True Fit carseat. I don't think it's available yet - coming out soon try amazon.com. It has a detachable head rest so that you can keep a
rear facing seat properly reclined for a younger one if you've got a tiny car. The head rest HAS to be on when child hits 22 lbs. But by the time most babies hit 22lbs, they're old enough and have enough muscle control that they can be more upright (still
rear-facing), which means that the seat can be reinstalled more upright with the headrest on and still fit in a small car.
Hope this helps.