If you copy an elite from a chapter you have not purchased, it will not be available for your use.
Heroes use this skill on their own, but only for a skill they can use. That means, of all available elites around the team, they will copy a skill of an attribute that matches their highest attribute and if they have at least 6 points in that attribute. On the downside, they use Mimicry whenever possible, not necessarily when they or the team needs it.
This skill opens up many interesting team possibilities when used with Echo chaining.
Heroes cannot copy PvE only skills.
Glyph of Renewal does not affect Arcane Mimicry unless the casting of Arcane Mimicry is interrupted.