A couple of questions (teleporting communions, fire gem use)

[Baalz]

1) Yes, the order is preserved no matter what throughout the whole game. The only complication is if they're coming from different provinces there is no way for you to tell what the order will be - but it will always be consistent if you paid attention earlier.

2) Entirely situational. Fire magic's highlights are flaming arrows, fire brands, shields of gleaming gold, lightless lanterns, incinerate, falling fires, and several really nice situational things like flambeauxs and rune smashers. You only need to go into fire if you need to go into fire.

[Rytek]

A few fire mages in your armys could spam resist fire somewhat lessening the vulnerabilty of protection and wooden warriors being cast.

[chrispedersen]

Early game flame arrows is kick butt for man

[Ironhawk]

Originally Posted by Baalz

The only complication is if they're coming from different provinces there is no way for you to tell what the order will be

Its better to say that there is no easy way to do it, rather than it being impossible.

The only way you can test ordering of units in different provinces is to temporarily give them move orders to a common province and then use the 'y' key to check their mutual ordering (before reassigning them back to cloud trapezee). Temporarily equipping boots of flying can make this process much easier. Of course, you cant do this to every unit you want to check - but the unit ordering is static across your empire. So if you test the first/last unit from province A against the first/last unit in province B, you will understand how their orderings interact. You can then chain this testing from one set of labs to the next to determine the full ordering of every mage involved, assuming they are all within 6 provs of each other.

While it is a fair amount of micro-management to do the above tests - the results can be invaluable. Allowing you to reliably land communions composed of mages from across your entire empire.