Here's how it breaks down re Scotty's uniforms:
Leaving drydock/impulse: standard
Warp drive/wormhole: Rad suit
Fix engines w/Spock & Warp 7: standard
V'ger weapons attacks: standard
After flyover tractor beam: rad suit
So, it's pretty consistent except that Scotty ran to put his rad suit on while they were flying through the cloud, apparently.
Interesting topic, I recently brought it up, too, in blssdwlf
's Enterprise WIP fan art thread
Apparently the standard uniforms / jumpsuits are radiation-proof to certain levels, like the one Scotty's assistant Cleary is wearing
(with the typical radsuit collar).
My guess: minimal standard radsuit gear in impulse "engineering"
In the warp drive "engine room" white and heavy radsuits appear to be the minimal standard.
As for the lower intermix and reactor levels, the red-orange heavy radsuits might be the minimal standard.
In TMP Scotty changes back and forth quite a lot, maybe he doesn't like to wear the heavy radsuit while in impulse engineering and it really took him some time until TWOK to wear this thing permanently.
Unless I'm mistaken it's not only the kind of radiation one is exposed to but also the period of time. As for Scotty he might leave the engine rooms to go to his office, be on the bridge or else (i.e. unlike the engine room work shift he doesn't get his daily dose of radiation)?
Same could apply to the helmets, the longer you stay it becomes imperative to put these on (alternately temperatures in the rooms might go below zero to optimize conductivity of the intermix components, so radsuits might also work as thermal gear).