I would think it would be ok to mix it. Are you sure the Pale isn't 2-row? Arrogant Bastard is a strong beer -- I don't think it should be too sensitive to base malt subtleties. If it were me I'd calculate the recipe with the amounts of malts I'm actually going to use, especially if the Pale is 6-row which is probably a little lower yielding.
My next two beers on deck are a porter and a stout, and I'm running out of my Maris Otter and my next 50# is 2-row pale. Since they're both dark beers I don't think I'll taste the difference, so I don't care when the changeover will be.