Just starting/old injury, any advice?

I am just about to get started with some gymnastic training for the first time in 21 years. I was almost competitive as a gymnast from 8-13 years old but for some reason as a kid I did not want to compete. Back then I was decent at rings I could do giants and HS nothing big strength wise. I sucked at pommels and was okay on high bar and vault, so so on floor. I am 34 and in relatively good shape thanks to two years of 8-20 hrs week training for motocross. I spend a lot of time on a road bicycle, concept 2 rower, the MX track, and in the off season I do a small amount of lifting.

However, almost seven years ago I destroyed my left shoulder. (dislocated, fully sepereated ac, broken head off humerus, damaged axillary nerve, lots of soft tissue and support damage.

My ROM is back to I'd say on the safe side 85% maybe a little more. I still ahve some nerve damage, I can tell by a lack of feeling on the skin in the area of my deltoid. Strength and size of muscle in the injured shoulder is under 15% of my good right shoulder. Basically my left shoulder looks and acts like the shoulder of a 12 year old girl with no exercise.

I did dislocate the left shouder a second time about three years ago, it was no big deal the ER put it back and I was back to riding MX in a little over a month.

Does anyone have any suggestions or advice about how I can approach gymnastics with my old injury and how much of a factor it will be?

All you can do is start slow and see where you go. Start with planks and the other pre-requisites and build up slow.

I had a client I worked with one time with a similar injury to yours (fully separated AC). We did a ton of mobility work for the shoulders as well as took things very slow. The exercises that we avoided all together (there was discomfort even with the easiest variations) was any dipping variations or pressing at that low angle. He responded very well to overhead presses and lots of pulling exercises from all angles.

In the end you the best thing you can do is, like slizz said, take things slow with lots of prehab work and make sure to avoid the exercises that cause discomfort (this might mean you have to fore go things like the muscle-up).