But I have yet to hear a really concrete definition of "kind".
You won't hear one.
It is quite obvious, when reading the Bible, that "kind" simply means all of the creatures that go by the same name, using the folk naming conventions from that era.
Is "Penguin" a kind?
No, it isn't. That's because no name had been assigned to penguins by that culture. Nobody involved in the authorship of the Bible had ever seen a penguin. Since there was no name for that group of birds, they were not a kind.
What the thought process, if any, involved in Creationist classification?
The creationist "kind" is not the same as the biblical "kind". To a creationist, the term "kind" is used for obfuscation and deception (particularly self-deception).
™ Version 4.2