I plan on zeroing my scope at 200 yards. I doubt I will be shooting further than 600 yards to be honest, but I was just wondering if having a 20 MOA and shooting under 600 would affect the adjustments of the scope or have any negative impact.
It won't have negative impact. Your zero will be close to a third of your scope adjustment so you will have 2/3 of your available adjustment for longer ranges. The final number will depend on what your scope adjustment range is. It helps to stay away from first and last 10 MOA's of your scope for accuracy purposes but again, that will vary on your scope quality. Either way, nothing to lose, only to gain by putting 20 MOA rail on.