An A1 with a stock inline reg would be better with a LP tank, some work with HP, some don't (they work, it's just impossible to chrono below 300fps).
Another option is to replace the stock inline reg with a CP reg adapter and aftermarket inline, I use a shorty CP reg on mine (pic:
http://www.chimpy.com/paintball/a1_1.jpg ) and run a HP Ninja tank with no problems. Again LiPs can provide the bits you need.
The A1 Fly comes with a different reg setup and works better with HP, although some people have reported problems getting the fps below 300 once they installed a Boost bolt.