Ok, I am making a retirement cake for my father in law, and I haven't ever made anything bigger than a 2 tiered 6" and 10". My question is, if I have a 6", 8", 10" would it be secure enough to make the bottom tier 12" or does it need to be 14"? Also, would it be advisable to assemble it tier by tier at the location, or better off doing it before hand?

A 6-8-10-12 configuration is extremely typical. With proper structural support, the cake will be just fine. You can put a 6-8-10 configuration on top of an 8" cake...it's ALL in the structural support.

I personally transport mine finished, but others don't. It really is up to you and what you are more comfortable with. If you transport it completely finished, I recommend that you chill it in the fridge overnight before delivery to ensure the cake is chilled solid. It makes transporting much more stable.