I would personally stick to a larger 12v SLA, like the 12/12 you mentioned. The smaller SLA's aren't rated for the amps a laptop pulls, so the discharge curve will be much shorter than what you'd expect for the amp-hr rating.
As far as your other question about the car adaptor, I'm not clear on what you're asking. "Any old car adaptor"? I would only use the one recommended for your laptop. If that isn't possible, you're going to at least have to make sure it will output the correct voltage and amp rating (laptops typically need 14-17v, with narrow tolerances). There may be some "universal" 12v/car adaptors out there that may work. You could hack the male cigarette lighter plug end and put a 4pin xlr or mini-xlr, but then you lose the ability to use it in a car (charging internal batteries, watching movies, etc). I guess you could hack a male AND female xlr into the power adaptor so you could put it back together to use in a car.
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