Well, you are starting with a dog you already said was aggressive to other dogs, based on fear. In hindsight, I'm sure you see that you should've overcome that issue before introducing another dog in the mix. But, what's done is done and Ruby is your baby too, now.
So, with that said, control the environment and work it out over time. They are not going to be best buds right now (and you need to accept that they may never be), but you can make it where they can live together in harmony. It will take a lot of work, but that's what we sign on for when we bring in a pet (s), right?
Control - you know Maxwell is food aggressive so don't feed them together. Give Ruby a safe environment to eat. Put their dishes in separate rooms (closed off from each other) and feed them. That's the first step.
Other than food, you will need to always monitor them. Never leave them alone together until you trust Maxwell with her. Make the time that they are together FUN = playing and treats! Woohoo! This will help Maxwell gradually begin to associate Ruby and her presence as a good thing. I stress - be patient with all of this. KNOW that it is going to be gradual and take time. There may be set backs, but go with it.
Monitor Ruby's interaction with Maxwell, also. She is only 9 weeks, which I must state for future readers - that is really too young to be taken away from her mom and siblings because they learn so many social behavior skills thru weeks 9 thru 12 (like bite inhibition). If Ruby bites too hard during play, another pup would give her a warning shot to tell her "hey, that's too rough" but Maxwell may react out of that fear based behavior and hurt her back. So just keep a close eye on that ok?
Some others will come along to give some tips and help. I'm not expert in behavior like some around here (I think of yorkietalkjilly and gemy immediately...hopefully they will see this and help).
You have what you have right now so don't give up. You made a commitment to both of these pups and they are counting on you. Help them work thru this. With work and time and PATIENCE, you'll get them living in harmony...and, hey, maybe best buds someday. If not, at least brother and sister.