"Baron Man" wrote in message . ..
can one log PIC if another person is flying.

Safety pilots can IF they are serving as PIC at the same time since
they are a required crew member.

what if you're an instructor.

Only if you are actually giving instruction. The FAA gets pretty grim
when people try to log PIC all the time they ride with their friend.
Make sure you log who to flew with and what type of instruction you
provided.

same for twins - if i have a ME instructor rating, can i
log PIC time in an airplane that i do not have formal
training in - but is similar cat/class - piston twin etc.

Only if you are actually flying. You can't give instruction in the
twin unless you already have 5 hours of time in the plane so you can't
give instruction.