Wocs, will I be able to stay in my field of artillery or will I have to take what ever is open?

Not sure if you ever got your answer on this or are still seeking it out, but my advice would be to google the warrant officer recruiting command website. They have all the information you need on what you need to have completed before you can submit a packet. I've looked into this a couple times myself and of the many requirements that might hold you back "having 2 rated NCOERs with section chief time"(not waiverable) and "Graduate ALC (all phases)"(not waiverable) are probably the biggest. So as your chief said and as I would tell my soldier, worry about now and keep your nose clean and buried in the 6-40, 6-50, or what every your "ARTY Bible" is.
Good luck
Also:
http://www.usarec.army.mil/hq/warrant/prerequ/WO131A.shtml

To be honest, it would depend on what you plan on doing, there are a few MOS's out there Aviation and some of the Cyber/Computer related fields that are branch immaterial. Keep in mind most active duty Chiefs are busy as hell, and if I had a dollar for every private who asked me "How do I become a Chief" I would never get anything done on weekend Drills. In todays Army, with the amount of information available at your fingertips, you can answer almost every question you have in a few minutes. I am not trying to discourage you, Chief's have things to do.