[@Cinderella Man]

To answer in order:

Yes, Alchimia users usually specify what kind of materials they can transform, but their range is much bigger than most other mages. E.G. A Metal Alchimia mage can change any kind of metal. 

Yes, you can use Alchimia as a method of fighting like in FMA. It would depend a lot of what you can transform, though. What I have thought of while making these is a metal alchimist able to transform metal they carry with them into weapons, or someone with magic related to earth, making structures out of even sand to attack, this last one imitating a more a la Avatar vibe. 

Nothing is off limits when changing. If you can create gold, then you can do it without restriction. However, it is not permitted to make gold out of nothing. 

Okay, this is more of chemistry and I'm not really that good at chemistry, so bear with me. To rearrenge elements or objects into other things, a mage has to go into molecule territory, separating or adding things to a formula to create new things, or simply add one thing to another and shorten the process of making some other material. As before, it would only be effective with the branch they have chosen. E.G. If you are going for metal, if you have iron and add some coal with your magic, you get steel without having to molt it, etc, etc. and so on. 

I hope that answers your question, if not feel free to ask me more.