Can "lead free" tips be used with leaded solder?
I would imagine that the only real difference would be the initial tinning on the tip.

Generally, you can use a lead-free soldering iron tip with leaded solder without issues. However, it's generally not a good idea to use a leaded solder tip with lead-free solder.

IIRC, there are issues with having lead impurities (e.g. a little tiny bit of lead) in lead-free solder causing the joint to become much more brittle then either type of solder on it's own. However, I can't find the paper on the subject at the moment.
I don't think lead-free solder contaminating a leaded-solder joint would be an issue.