I think you'd be fine with cold rolled steel. I think that a plate that size may be kindof heavy for a taig mll but that doesnt mean im right. Under the kind of use your talking about I doubt youll wear out any of the steels your talking about.
When you grind the surfaces. make sure you shim them so that whatever warp is in the plate will be removed.
As for bold size Id go with #10x24 there mor then strong enough thats what I used when I had a sherline.
You might want to consider pocketing the back side of the plate to lighten it.