Are there "guidelines" or "rules of thumb" for how thick a blade should be/length? For example; bowies. Does a 8" blade need to be 1/4", or more, or less? I've been told that since steel was expensive, back in the day, that the "bowie" was similar to our present day butcher knives? Is that so? What are your preferred thicknesses for your different types?

The thicknesses for "bowies" back in the day probably had far more to do with the availability of the steel than the person making the knife.
There are many design factors to be considered when making ANY knife.
I try to keep my larger Bowies in the .260" - .270" thickness.
It depends on length, handle material, butt cap or not, etc. as far as balance goes.
How much distal taper depends on end use of the knife.

Personally, I like 1/16" for kitchen knives, 1/8" for small to medium hunters, 3/16" for medium sized camp knives, and around 1/4" for big choppers. Of course there are variances, since I forge most of my blades, and I'm not necessarily skilled enough to get the exact thickness I'm aiming for every time.

My preferences are pretty much the same as Todd's, though I'd leave most kitchen knives a tad thicker.  Capers and small skinners (~3") around 3/32" thick as well.