1) Yes, no problem. Just make sure you get good fusion and no slag in the middle of the joint.
2) Unless specifically told to do otherwise, always electrode Postitive (reverse polarity)
3) You should not tig over in this case. All you will do is overheat the tubing and make a large heat affected zone, making the area just outside of the weld weak and brittle.

A 6013 will run well IF you clean the metal slightly with a grinder first.