When bareshaft tuning and you finally get it tuned the way you want do you have to use the same weight broadhead? Or do you have some flexability with that without changing the spine to much?
Adding or subtracting weight will affect the spine of the arrow, and in fact is one of the best ways to adjust and fine tune your set up. You probably won't notice small differences, but I try match my field tip weights to my broadheads as close as possible. There are a good variety of field tip weights available, plus weight washers, etc. that make it pretty easy to do.
Yes and yes.
Generally you would want to tune and use the same weight broadhead. There is some flexability though depending on the shaft you use and the bow you use. If your bow is cut to or past center, you can use a stiffer shaft and heavier broadheads as a rule. You will still want to tune your bow to these however because such a change may require a change in brace height, nock point or both.
Your life will be much easier if you stick to the same weight as your field points. ;)