I'm not sure I can explain your questions without writing all night.

I'll try to explain spine in a sentence or two as I understand it.
The stiffness of the arrow= spine, too weak and the arrow will wobble and kick on it's way to the target because the pressure placed in the nock is too great for the arrow to absorb the energy, the rear tries to flex towards the front on its way down range.
Too stiff isn't as easy to do, but can be done. Not sure how to describe too stiff, just know thatI ran into it this year.

As far as arrows go, I'd recommend Any Beman or Easton in the 340 spine rangewith 100 grain tips.
If speed is your all out concern you might get away with 400 spine, but might find it too weak for the energy the Allegiance produces.
In the end you may be talking 20-30grain difference between 2 like arrows.

Shoot the 340's and be happy with whatever speed they produce, the Allegiance will provide plenty enough for whatever arrow you choose.

My question to you is how did you buy a new Allegiance and leave the proshop without discussing arrows with the purchase?
Did the sale end with the money transfer? Most shops I frequent would at least go over suggested arrow combo's before you left the shop.
The proshop you purchased the bow from should have a nice fleet of arrows ready to test shoot & buy, if the spine proves to be wrong you could swap them out before you left with your new arrows. That is the best way to go when trying to find what a new bow likes IMHO.