None of the responses address my question.

What is the DOC to do LEGALLY?

I would think putting an 18 year old girl, regardless of her self perceived gender, into a men's prison would be at the very least, dangerous.

They would open themselves up to a lawsuit when violence happens.

And what precedent would it set for future cases?

A man facing time in a federal maximum security institution could claim he's a lady inside and be sent to a women's prison.