alpha and beta have never been given strict definitions. in the age of apocalypse, it seemed that alpha was a designation given to those apocalypse deemed fit to survive. beyond that, there is really no classification that is a constant through all confirmed alphas or betas, with the exception that their powers are NOT limitless.
a couple sentinels said storm was a possible omega. that's no different than wisdom saying chamber might have omega level power. she's still an alpha, despite never explicitly being labelled as such [as far as i know.]
on another note, forge is a confirmed alpha. and his power is hardly powerful, when you think about it. so, again, there isnt any strong themes running as constants through the terms alpha or beta.