Artist acrylics differ greatly from hobby acrylics, which differ from craft acrylics. And artist acrylics actually have at least two grades: standard [or "student" depending on brand] and professional. The difference between those two grades is the amount of pure pigment versus the amount of filler. Professional artist acrylics will have the richest, most vibrant color and best paint quality, though they will be pricier.
However, if you $6 for a tube of art acrylics, you're getting a tube of mostly paint. If you get a jar of hobby acrylics for $3, you're getting mostly water. So, if you are willing to take the time to learn how to properly prepare art acrylics for miniature use, I would highly recommend that over hobby acrylics in most cases. It's a better value, overall, as a single tube of pro-grade paint could last you an entire army.
I wouldn't recommend craft acrylics for anything ever, though. #P
Thinning them can be as simple as adding water or alcohol.
I've never heard of anyone adding alcohol to acrylics to thin them. I would imagine that would just make the paint dry out quicker than it does already.
Typically, I just use water, myself. I've used gel medium as thinner for paintings, but I haven't tried that on a miniature.