New rescue snake, I need some advice (Sorry it's so long)

I just got a new snake yesterday off of a facebook group about snakes, the woman I got her from broke her leg in January and she said she hasn't had any time to take care of her. She came with everything the woman had for her, but that only included a 20 gallon long tank with a sliding screen lid, a red heat lamp and some decisions and half log hides. I already have a ball python, so I know all their needs and everything and I know I need to get a under tank heater tomorrow, I already have extra thermometers so I put those in right away. She is very tiny, smaller then my male of the same age, about a year maybe a little younger. But she is really aggressive, hissing whenever I take her lid off and constantly in the strike position. Is there anything I can do to calm her down? And I normally feed my male (Juju) every Wednesday, should I feed her as well tomorrow, or wait a week to let her settle in? She said to tried to feed her a f/t mouse last week but she struck at it and never ate it, so I assume she didn't eat anything last week.

Just from your initial description, I'm thinking you have a hungry, stressed snake on your hands. Kudos to you for trying to give it a better life. You mentioned you already have a snake so do what you can to mimic an ideal enclosure. FYI, those half logs arent ideal for the snakes. I suggest letting her get used to her new environment by leaving her alone but try feeding her. Only open the enclosure when needed (changing water, cleaning, etc). I suggest not handling her until she's had three successful feedings. That way, she'll get used to her new surroundings, hopefully realize that you are no danger to her and see that you provide food. That may calm her down. After a few successful feedings, then try picking her up for a bit.

Anyhow, this is how I would approach your situation. I currently have a Craiglist "rescue" snake myself. Not aggressive like yours but mine was VERY skiddish and nervous. Its getting better now. Oh, and most importantly, keep the new snake separate from your established snake for at least 3 months.. minimum. You want to quarantine the new guy from your snake just in case there is a health issue you cant immediately see. I'm sure other members will chime in with some additional thoughts. Good luck and please keep us posted. We like pics and updates.