Can we install kong if we alredy have a nginx running in in our system?
Yes, but you cannot share same ports they are bind to. But Kong is Nginx, so one option is just move everything in your Nginx to Kong, and use custom Nginx template with Kong:
So both will work, side-by-side or moving to Kong only.
Or you can have them in layers, e.g. nginx proxies to kong or kong proxies to nginx.