There is no way you are getting a car seat into a Chariot Cougar. We have two infant carseats -- a Graco Snugride and a Maxi Cosi Mico -- and both are far too long and wide to fit into the Chariot. If you have a double Chariot you can probably manage the width, but that won't help you with the length.
Maybe they've redesigned. Try before you buy if you are insistent on this. I don't think it is going to work. We were told by friends that it was easy to do, but those friends had Burleys or other brands.
ETA: we do have Chariot's "Baby Supporter," which is just some extra padding to make the seat smaller for a younger kid. We used that until last week -- my daughter is 22 months but quite small, not much bigger than your little guy.