Hmm its interesting to think about ARN/SMN being from Limsa. I'm not sure if it really fits. Most likely to me, it would seem more feasible for ARN to come from Ul'dah, because they study artifacts and incantations. For them to be accompanied with pirates and men/women of the sea just doesnt seem to fit. But yes, not many classes delve from the Sea port city of Limsa Lominsa, even though all but one DoM/DoW doesnt reign from Limsa, many DoH do.

As for the OP, I'm pretty sure it was confirmed Arcanist will be available only for release, where as Summoner was said it might not be ready for release. They werent specific as to anything regarding Beta, but you can be sure, once its ready you can pick that class at character creation screen.

(http://www.luxarcana.org/forums/mmorpg/topic6373.html bottom of the page under Misc.)