What is the best time to go to this zoo, maybe late afternoon because of the heat? And is there only one fee schedule or do they charge differently for foreigners? I hold a retirement visa and a thai driver license if that has any relevance.

go early in the morning, animals most active, less crowds too.

prices, if I recall correctly are dual. However if you have a WP you get the Thai price which is very cheap 40 baht or something. 100+ baht with foreigners who are just visiting thailand. Not sure about discount for retirement visas.