I have 6 1500lb rated castors I was thinking about putting on the bottom of the table to make it slightly mobile... basically roll from one bay of the garage to the other. I see a lot of folks using leveling feet or just steel legs. Is there some big disadvantage to having it on castors? Its gonna be a dam heavy table so the movement of the Y axis shouldnt really move the table around.

I need your wisdom..

Thanks,
DeviousMW

When you have your machine on casters, the inertia of the gantry moving around can shift the table on the casters, at least during rapid movements of a heavy gantry. This can also shift your workpiece.
Casters are not a bad thing, but make sure you can retract them or lift the table off the casters.