you can do this, but there's little idiosyncrasies. first off....r134 works TONS better than r12.....and so far it seems better than the new r123hf. it is sensitive to temp/humidity though. you pretty much need to see the pressures. I helped johnnyb out yesterday. on an 87degree day with about 78% humidity, his pressures were about 48psi and 145psi low and high respectively. those were just a hair low, and the vent outlet temp was about 64 or 65 degrees. I added a couple ounces, brought the pressures to about 51/155 and dropped the vent temp into the lower 50's I think it was.
if you overcharge these systems even just a few ounces, they'll start to get warmer. overcharge too much, and your high side will get high enough to pop the blowoff valve.
if you use one of the small cans, hook it to the system, open the valve, let it go for just a couple seconds, then close the valve and give it about a minute to see what the vent outlet temp drops to. repeat and rinse. just add tiny amounts each time so you don't overcharge.
the ones that come without the hose, you need to buy a hose assembly to tap into the can with