For example if I had many years of experience in kick boxing and I was very good and I started doing TKD would I start at the beginer level? OR if I was a black belt in BJJ and I started judo would I start as a beginer or would they put me at a apropriate belt?
Most likly you will start as a beginners since that is what you really would be. But in judo and bjj they are som similar it might depend on the instructor. They might not start you at the beginning.
Cause you should know their proper stances and you might learn one or two things you didnt know, that way you can complete your own arts and decrease your weakness
It is their decision of how fast you adapt to things. I do kick boxing too for 6 months and TDK and they put me on a white belt LOL i learned alot by just being there cause the ranks differs from each other right? That means the teachings are also different. It is on their hands to decide if you mastered each level's teachings