I'm talking about the Flame Cloak, Frost Cloak and Lightning Cloak spells here. I know that Chain Lightning is hazardous to nearby allies by what I've read about Marcurio. And J'zargo's scrolls had a similar effect if the spell accidentally touched a follower. So, if these two are anything to go by would I be unwise to teach my destruction mages these spells? The same question goes for the storm spells. Are they at risk of hitting followers or just enemies?

I believe that cloaks harm only hostiles (including brawlers). Not sure about storm spells, since I haven't actually tried one yet.

I used the Cloak spells a lot with my last character, had followers or other quest companions around a good deal of the time. I did not see them taking any damage from the cloaks when the spell was active. I *did* make an effort not to get too close to them if I could help it, and from what I could tell they tended not to get right up in my space either if I was flaming/frosting/sparking. I never had an issue with a friendly going suddenly hostile or dying (AFAIK) because they took damage from my cloak.

That last playthrough was my first experience with those spells and I will definitely be looking for them again as they were incredibly useful, especially if I was adventuring alone and found myself getting mobbed by melee enemies.

Chain Lighting is designed to bounce from target to target, and unmodded it will not distinguish between friend and foe. The "issue" with J'zargo's spells was that they were defective (or deliberately designed to remove a potential rival) and exploded, causing damage to their user and anyone else in the vicinity. The Cloak spells don't work like that, they're more of a static protective 360 degree barrier that does damage to anyone who gets in your space. They are passive in that sense, they do not "reach out" and look for a target or do collateral damage to anyone who is not physically in contact with you.