Good day to you, allow me to just help get the ball rolling here;
Does the flight time include hours flown in a cessna 150 or cessna's for that matter? for example hours that you gained during your training as a student pilot all the way through CPL/IR/ME ratings?
500 hours is grand total (minimum) on your experience. This is not type specific (for obvious reasons).
Would being a CFI on a cessna be enough to apply as an FO for the atr?
Having a CFI credential does help. Again not type specific. Although having the experience in complex aircraft, flying environment, more advanced avionics, multi time, night flying, actual IFR exposure, ...etc etc etc would determine IF your minimum requirements are deemed acceptable for ATR FO duties. In 5J, they will initially consider ATTITUDE as one of the fundamental criteria. Then will train you to proficiency. They will not hesitate to send you home even on the first week of basic indoctrination if justified by your attitude and performance.
Or do the hours have to be earned elsewhere? Such as flying for charter companies?
'Real-world' flying experience will get you noticed. Example, flying command in charter/VIP flying, Regional experience, Freight flying (like the guys doing the Caravan run in Subic Naval base, though that is turbine)... etc. 91 and or 135 time is good.
I hope this helps. good day!