Having welded and doing most types of metal work for the last 16+ years professionally, I would strongly suggest you NOT use a mig welder for building bikes. It is possible after doing many tests and you might be able to use a dual shield welding wire and get lucky but as unter mentioned you have the distortion problem and then some. Either way if you do plan on mig welding it, plan on repairing it over and over with the hopes of not having a bad accident.
I would sell the mig and get a tig welder. You can tig weld just about anything but you can't mig weld just about anything. Tig welding is more structurally sound and controllable. Also you can get the right type of filler rod for your application (more options). Tig welding is tried and true. Hope that's some good advice! Good luck and be safe.