Aeromexico: You need to be a mexican national or legal to work in Mexico, with at least a Commercial ticket issued by the Mexican FAA. After that you need to affiliate with ASPA the union representing those guys. Before going to aeromexico you need to enter Aerolitoral the regional of aeromexico (there is a flowthrough to aeromexico) work there for 4 to 5 years flying EMB135 and then as the major needs pilots they are brought directly from the regional (no exams or any kind of hassles, direct entry).
There is a bid on the union to enter the airlines and as you can imagine, sons and daughters of Aeromexico pilots have the upper hand.
As far as flying time as long as you know the right people and have a commercial license with ME and instruments you are good.

The new low cost airlines Volaris, Interjet, Avolar, and soon to come: GOL Mexico and RyanMex they like pilots with previous part 121 experience (A320, B737 exp) and knowing somebody inside to recommend you.
Hope this helps