By "optional" I mean "you can complete the game without hiring".
In Mass Effect 2, you could finish the game without hiring Kasumi Goto (a DLC character), and I think Grunt and Legion are optional as well (you have the option of not animating/reactivating them), though all others were mandatory.
What about Mass Effect 3? Is it possible to finish the game without hiring one of the characters? (I'm guessing probably not, but I'd like to be safe). I'm not talking specifically about DLC characters, but any character that is possible to miss.