air bottles tend to come in two variations, a steelie or a fibre wrapped. steelie is the straight up can of steel as you'd normally find at a rental field. a fibre wrapped is as the name says..
fibre wraps tend to be a lot lighter than steelies, though a bit pricier.
all bottles tend to use a standard thread size and will usually fit all markers.
ratings? there are 2 fill sizes 3000 psi and 4500psi. a lot of players prefer 4500psi tanks as they can accept a bigger refill if it is offered, whereas a 3000psi tank will only accept 3000psi.
Inspected, a company called HPac seems to be the main that offers testing and inspections, however if you buy new you get about 5 years worth of life in it before it needs testing.
Safety, if you are unsure as to how to refill, use an air tank someone at your local site will show you, i also believe one or two tournaments offer a quick training session on how to safetly handle tanks and give those that attend a pass to show at future tournaments.