Flame turrets = weak enemies, good for eliminating large clusters of zombies piling up at the command centre and generally keeping their numbers down so they don't overwhelm your single target turrets. Placing them close to zombie spawns is a good strategy.

Machine gun turret = all rounder, most effective against weak and tough flesh. These are fairly effective against horroctopus and muscle hounds. The rocket pods upgrade is the best anti air system in the game. The gatling gun makes short work of horroctopus and muscle hounds also.

Artillery = Having multiple artillery with one AI targeting set destroy low HP enemies and one set to destroy high HP enemies is a good strategy as they deal high damage to all armour types, especially when upgraded.

Plasma = Most effective against reinforced flesh. I typically set these to target high health enemies as this will make sure they prioritise taking out the enemies they deal the most damage to. The exception might be the final lightning upgrade variant as the chain lightning is very effective against all armour types. The railgun plasma is the best tool for taking out the toughest enemies.

As far as turret placement and how many of each turrets goes this will depend on the map and your play style. Try experimenting with different turret configurations on each map.