Rick,
  In answer to your questions:
1.  Whether I refill my oxygen or not depends on how much is in the bottle and how long a trip I'm going on.  If I'm only half full, but not going far, I don't bother.  For longer trips, I'll top it off.
2.  I believe my bottle is 22 or 24 cu. ft. and is aluminum.  I wanted something low enough in profile to fit under the floor in my baggage area. The Kevlar bottles are really nice and much lighter, but they're larger in diameter and wouldn't fit where I wanted to mount my bottle.
3.  I refill the bottle myself.  I bought two large welder's oxygen tanks, a cart to hold them and wheel them around and got Mountain High's transfiller system.  The whole setup cost me about $800, but it will pay for itself within two or three years.  I just roll the cart up to the baggage door and use the filling port mounted inside.  I've never had to remove my bottle yet.  When my fill tanks run low, I go to a local AirGas store and exchange them for about $20 each, which is less than the cost of one refill of the tank in my plane.
4.  The white bottle in the photo is a halon fire bottle plumbed to two nozzles inside my cowl.  One shoots over the cylinders, the other over the accessory section.  I dount if it would do much good in flight due to cooling air blowing the halon right out but it would probably do a good job in the event of a ground fire.  I mounted it in back like that strictly for weight and balance purposes.
    I got the fire system from Safecraft (http://www.safecraft.com/indexAviation.asp).  If you're going to put something like that in your plane, I highly recommend this system.  The plumbing to the firewall is aluminum tubing and from there it goes to the nozzles via flex hoses that I made.  It's activated by a T handle on my panel that pulls a cable attached to a pin on the bottle.  To route the cable, I bonded in a long piece of Nylaflo tube down the side of my fuselage that the cable slides through, so I can easily pull the cable out and put it back in without removing any side panels.
  One note: the bottles are designed to be mounted head down for gravity feed of the halon.  If you want to mount the bottle horizontally like I did, tell them and they'll put a weighted flexible feed tube inside the bottle so it will still work in that position.
  Let me know if you have any more questions,
  Skip Slater