The Chicco KeyFit 30 is a good choice at 16.75 inches wide. It is not quite the slimmest car seat you can find, but it is an excellent car seat overall that should meet any reasonable requirements a parent may have for a car seat. It’s better to use the base (latch system), but if you can not use the base this car seat will allow you to use the seat belt instead.
It supports children between 4-30 pounds. I usually like car seats that support children up to 40 pounds so they can stay in the rear facing position for a longer time period, but 30 pounds isn’t half bad for a car seat this narrow. 30 pounds isn’t too bad for the upper limit. It shouldn’t be a deal breaker.
The car seat has multiple shoulder strap positions making it a great for your kids as they grow older, but the crotch buckle position is limited possibly making it uncomfortable for bigger kids.