For pathfinding: Unless your RTS game is alot different than most, you may want to consider exploring the Astar Algorithm. There is a TON of information on it available via web searching (also try graph algorithms) so I'm not going to bother with giving you just one URL to go to. Look it up, read about it, and implement it, and you will get better pathfinding.
For deciding where to build: Consider how YOU play the game. Where do you decide to build a particular building? And why did you decide to build it there? What were the terrain or map characteristics of the site that you chose? Basically, answering those questions will reveal to you the influences that you used to decide where to build that building. If you can identify those terrain and map influences that you use, and devise a system of expressing them within the map and terrain that you use, then you can use an AI technique called an Influence Map that will help your AI to determine the best spot on which to build. Again, there is alot of info on the web about Influence Mapping. Do a web search on the subject and you should be rewarded with something to read and learn from.
Most important of all, good luck.
There are too many questions in your post for me to answer them all at once, so let's just try for a couple of them.