Is chromoly stainless steel? Or are they two completely different things?

Mine certainly stains. Different.

Not stainless

its an alloy steel with its main alloying elements being chromium and molybdenum and a few other minor elements thrown in

stainless is an alloy steel too but it needs to exceed 11-12 percent or so chromium content to be considered a true stainless steel again other alloying elements make other types of stainless steel for example nickel

Am I right in thinking though that standard cromoly has a certain rust resistance over mild steel? I've had a couple of mild steel bike tools start to rust in days (just from the moisture in my hands I think) whilst an old pair of cromoly forks that were sat in a damp shed under a drippy spot in the ceiling for year or two with a completely exposed steerer didn't show a sign of rust.It certainly seems more resistant to rust.

Yep, generally speaking cromoly has a greater resistance to mild steel. Though I guess there maybe circumstances where it can rust very quickly. Likewise some grades of stainless can rust incredibly quickly if you can bits of iron embedded in the surface.

for all intets and purposes cro-mo is as corrosion resistant as mildsteel
it will rust just as fast as mild steel
your steerer is probably clear coated or has a protective plating
possibly etch primer applied before the colour paint on the bottoms
add a bit of stray grease and thats possibly why they didnt rust
try rubbing a bit with some emery paper or a file
then put them someware damp and watch the rust form
the salt from your sweat will be speeding up the tool rust
good quality tools should be made from tool steel yet another alloyed
steel but it still rusts