Are one arm pull-ups and one finger pull-ups bad for your shoulders?
All pullups risk injuring your rotator cuff (which can be very nasty and takes months to heal fully) if you do not engage your muscles the entire time and end up doing a "deadhang" where you hang straight down with your ear pressed against your arm.
Obviously one arm pullups are more dangerous in that regard since they are harder to do correctly, but if you work your way up slowly using various forms of assistance until you can actually do them without deadhanging, there's no problem.
One-finger pullups are a different beast - they also put extreme strain on the tendons and pulleys in your finger, and those gain strength far more slowly than muscles. Many a climber has ended up with a ripped pulley or tendon by training his finger muscles until they could hold forces their tendons/pulleys could not.
Overall I'd say it's not worth to risk that kind of injury just for a flashy trick.