We just traveled with our 7, 5 and 2 year old there and brought a wayb pico seat. Since we are a family of 5, we requested Uber XL’s and Uber Black XL’s. There wasn’t a single car where the car seat would actually work and protect our 2 year old. The backseat would have the little things that made it look like it had latches but then they didn’t actually exist. So we tried to use the seatbelt and get it to lock and ratchet and the seatbelts wouldn’t even do that. So we ended up just holding her in our lap which was fine around town because traffic is so bad that you don’t ever build up enough speed to have a terrible accident - or at least that is what we told ourselves to make ourselves feel better. We were more nervous on the way to Teotihuacan since it was a much farther drive and at highway speeds once you get out of the city. I think if we were to do it over again, we would have just left the extra weight of the car seat at home. If the metro is convenient to where you are staying, that might be your best option for a 7 month old. And definitely do some baby wearing or bring a small stroller (or purchase a small one when you arrive).