I think you are going about this the wrong way. There is no way to properly teach you what you need to know to decide the kinds of questions you are asking without time and effort on your part. What you are asking for is experience and that is not easy to convey because the answers are not always that clear.
What you should do is to find a location which has been in business for at least 4 preferably 5 years and where you like what they say and how they answer your questions. Then trust their judgement. There are many good locations that have not been in business for at least 4 years but it is often difficult to tell which ones are good and which ones are not. On the other hand, bad operations don’t survive 4 years.
As far as cooling, the cryogen cooling from Candela was used because in the beginning all you had available for an alex was cold gel and the cryogen spray. The Cynosure machines had the ability to vary the pulsewidth so they were usable with cold gel. The Candelas on the other hand were fixed at 3 ms which is a very short pulse that is more risky to the skin and needed a more consistent cooling system than cold gel. With all changed in 2000 with the advent of the Zimmer cooling system. Studies have shown that the cooling of the Zimmer units (down to 0 degrees farenheit) is much better than the cooling of the cyrogen, just because it is on the skin much much longer.
Regarding the lightsheer diode, it is limited by its inability to provide high power in a short pulsewidth. It works good on coarse hair but has a tendency to make hair finer. But as it gets finer, it becomes more difficult to treat. You reach the point where it is almost impossible to remove the remaining hair, yet this is hair that will respond to an alex or YAG which can get a pulsewidth down to 3 ms.