Yes and no to all of the above including the matched pair idea. For the most part chainlinks and snowflakes should be alright together, there are exceptions of course at times. As far as the GDM's go I have at least one in every one of my tanks except for one of my dragon tanks with no problems. A lot of which moray will go with which depends on a lot of factors like tank size, amount of rockwork and how its arranged, size and type of eels involved, other fish in the tank, length of time one eel may have been living in the tank prior to introduction of another, and of course the individual eel involved as they do vary, etc.
Safest bet of course is alone, however I have been keeping multiple morays together for a substantial length of time with one minor incident ever.