I have recently added in new aircrafts and flight plans of various airlines flying in and out of WSSS, using traffic tools and the flight plans downloaded from this website. HOwever, there is nothing wheni started the game. No new aircrafts or flight plans.

Questions: Do we need to add on the aircrafts for our AI traffic or simply just conmplie the flight plans and trafics and everything will be done for us?

Also, keep in mind that if you have a large number of flight plans and aircrafts for AI, it may not be a good idea to use flyable airplanes. By flayable planes I mean very well designed models as they could reduce your system's performance. Full moving parts mdels for example is of no use when the airplane is to be used for AI only as well as cockpits. I don't know which website you are reffrering to bu t I would suggest you the Project AI official files which includes real life airlines (a lot of them) with real flight plans and the related airplanes to be used for AI (you can also try to fly them but they come without a cockpit for the reasons I mentioned above). Their website is http://www.projectai.com/.
Once you have their installer on your PC you just need to download the ailines' .pai files and install them via the installer. They also provide TTools and other utilities for removing the default FS ATC (Landmarks, Orbits, Soars etc.) as well as for customizing the GA traffic. You will also find unofficial files there but in most cases they do not contain the airplanes as well.