This question of improving the AI comes up regularly - search the forums for "enhanced AI" and similar phrases.
However the bottom line is that AI isn't AI at all, it's all programmed. If you are not happy with the way BIS programmed it then its up to you to use the tools provided - waypoints, triggers, scripting, etc. - to make things the way you want.
Calling reinforcements is particularly easy to achieve using scripts and I have seen a couple of reinforcement scripts knocking around the place, though I couldn't tell you where. Remember that the AI can't make a decision to call reinforcements: the decision must be made by you the mission designer.