Brake Drum Question

If you are putting new shoes on for sure put a new drum on also.

 

I think the general rule is 2 sets of shoes to 1 set of drums.

Those look pretty bad to me, I’d replace.

 

I've seen worse. Its called heat checking and is a relatively normal occurrence.

 

Those heat cracks probably do not warrant replacement.
Drums are marked with a maximum diameter for reuse. Often 16.620 inches ( about 16 and 5/8's ). Does not look like you threw any money away.

 

Can you replace just one drum?

I’ve always done them all at the same time or at least in pairs

 

You could, so long as it's not on the steer. I wouldn't myself, but on the drive or tandem you likely wouldn't notice that one corner was doing a slightly better job than the others. Up front you'd be wrestling the wheel every time you made anything other than the lightest of brake applications.

 

Only in a bind. That causes unequal braking. That is a bad thing in the rain. Snow and ice are worse.