When can I use こいつ, あいつ, etc?
I’ve seen that they can mean he/she/this/that in various contexts but when is it appropriate to use them versus 彼 or 彼女？
Roughly speaking, こいつ, そいつ, and あいつ mean "this/that guy", except they're gender neutral. Essentially, they're colloquial, and slightly rude, variants of この人, その人, and あの人, which mean "this/that person". 彼【かれ】 and 彼女【かのじょ】 are equivalent to the English "he" and "she" respectively, but they're relatively distancing words; they generally carry the connotation of "that man/woman over there" or "this man/woman neither of us know well". In most conversations, if it's clear from context who you're talking about, you can just drop the pronouns completely, but if it's not clear but both of you know the person by name, just use their name + さん/くん/ちゃん and you're good to go.
Hope that clears it up :>
If I am not mistaken, one thing to add to Ranquil's answer is that 彼 and 彼女 can have the respective meaning of "boyfriend" and "girlfriend", something never conveyed through the series こいつ, そいつ, and あいつ.