Is flying Army fixed wings a possibility for a first assignment or do you have to qualify for it after flying rotary first?

There are flight school students who go from Primary straight to Fixed Wing track. So, yes, it is possible.

Technically you will learn how to fly rotary first because WOFT is rotary wing primary flight school.
Then, it's needs of the Army. And since fixed wing makes up less than 10% of the total aircraft inventory, and because that pipeline is backed up with rotary wing qualified warrants all with years of solid experience, the chances that you could get 155A guaranteed off the street are very slim.

It generally always comes to need of the military first but when it comes to this even more so considering how few fixed wing pilot positions are in the Army.