Bite incident tonight that lead to learning the hidden history of my new dog

I write a lot so this will be a bit of a read.

So my family got a new dog a month and two days ago. (He is mostly meant to be my dog but my family helps with stuff like bills and we all live together). We found him on Petfinder and we kinda rushed into taking him home as he seemed pretty great and his foster gave us bad vibes. He was said to be good with other dogs, partly trained and main issues being hyperactivity and some jumping and nipping as he is young (2) and didn't really grow out of that yet. New dog (Jackson) is a Pomeranian pitbull mutt mix and is around 25lb and as such is much bigger then our elderly three Chihuahuas that we already had at home (who are 5, 5, 10lb). Jackson lived with other dogs at the fosters and his adoption profile said he was good with others. For the first few weeks he was very passive around the other dogs and while there were some small incidents with our one Chihuahua who is very reactive overall it seemed like this might be a mild issue solvable with some basic training. He was much more than just nippy with people but it still seemed overexcitment/unsure how to properly give affection so again it seemed like it might be able to be solved with some normal training and if it wasn't we would deal with it at that later point.

Well on Christmas Eve Jackson got overwhelmed and stressed (we didn't put up our Christmas tree till then and so was very active and moving stuff in the living room) he got into three fights with our reactive Chihuahua two which Jackson started (we know in hindsight we should have isolated them instead of temporary separation for a hour or so but we'll that's hindsight). The third fight Jackson bit the Chihuahua on the neck and caused a large cut. It was shallow and we were able to treat it at home but it rattled us greatly. The day after that my parents left on a trip and took the 3 Chihuahuas with them and it was just me, Jackson and my brother for a week. Jackson was very well behaved during this time. Well yesterday my parents got home and everything seemed ok. Jackson had started a couple days before they left to hover and stare at two of the Chihuahuas and that seemed a bit worse at their return but not to bad.

Today we basically celebrated Christmas as we didn't on actual Christmas. Jackson got lots of new things and he got a special toy from me that seemed perfect for him. He seemed instantly obsessed with it and while before there was very minor occasional resource guarding over treats or toys this was a full blown resource guard. Our littlest Chihuahua (Skip, who never had a negative interaction with Jackson) came towards me and hence within two feet of Jackson and his new toy and so Jackson attacked him. Chaos ensued as I got him to release Skip and held Jackson. Skip backed away and initially held his head in a weird way that made me fear his neck was injured. I was so scared that he was fatally injured. It turns out later once he was checked Skip had a small puncture or two on his back (pinprick basically) and wasn't hurt. But Skip is very dramatic so when he was released and backing away he was screaming and acting like he was severely injured. In the seconds after Skip was released my mom told me to go take Jackson outside away from the area. The path to leaving the living room had been blocked and I was disoriented trying to figure out how to get Jackson out and I tried to pick him up twice in the wrong way (basically a way that was awkward and made me super vulnerable to attack) Well obviously due to the situation Jackson was freaking out and bit me on the mouth causing two cuts separated by my lips. I let him go obviously moreso to quickly apply pressure instinctively to my injury. My mom grabbed him and held him in a more secure manner sitting on the floor and slowly the scene de-escalated as I got a muzzle and harness to put on him and Skip was looked at and I also got a towel cuz I was bleeding quite a bit. Anyway Jackson took a long time to calm down was then taken on a walk and then isolated in a room when he was brought back in. It was decided I probably don't need stitches but a scar is likely to occur given my skin scars if blood is drawn most times.

While everything was cooling down my mom messaged the foster for Jackson and they revealed for basically the first time that they didn't let Jackson alone with their dogs as he was stalking and aggressive and had severe jealousy and resource guard issues and that most the time she didn't even let him be with her other dogs when she was there and basically this person who had downplayed the puppy like nipping reveals that Jackson has dog aggression and an extensive bite history with humans.

We are all kinda overwhelmed and conflicted and uncertain now. I love Skip so fucking much. I almost don't care at all I was injured just the moment of fear I had that Skip was fatally injured has me all fucked up. I'm still shaken badly even though he very much seems uninjured and fine. We went and saw Jackson and adopted him because he seemed like he didn't have any major issues. While we have had Chihuahuas before with behavior issues we never had a dog before that could actually do some damage. We got Jackson's partly cuz we lost our youngest cuddly sweetheart of a Chihuahua a month before getting Jackson and we really needed something to fill that hole of affection (our 3 Chihuahuas left don't cuddle and are not soft and affectionate) This is the first sort of pet that's mine since I became a adult and just I was considering a dog I thought I could make be a sort of therapy/ESA dog not a dog where we are conflicted cuz we don't want him put down and we are afraid of that happening if we try to re-home him but there is a legitimate fear about him killing one of our other dogs and so now we have to put all these measures in place to be cautious and safe and just I feel very loss.

We are going to look for a behavioralist and we have obedience training already paid and scheduled for later this month. My mom feels too bonded to Jackson and doesn't believe that he will be safe if we try to re-home him and wants to try everything even if it is going to completely change how we do everything at home so that there is constant supervision.

I personally am very loss. I'm Jackson's 'Mommy' and currently he is laying next to me in bed sleeping as I type all this. I am starting to like Jackson but it's just been a month so I don't feel a strong bond yet and I just love Skip so much and I can't live with it if I put him in danger. Before this I already would have paranoid dreams of him getting hurt. Now I'm just idk I understand why Jackson reacted as he did and I don't hate or resent him or anything but I just don't trust him at all anymore and I'm so anxious now and worried I'll feel that way for as long as Jackson and our Chihuahuas are in the same house. Also while I'm not opposed to increased responsibility of training and supervising Jackson it very much is not what I thought I was signing up for and just idk I need someone to help me feel less overwhelmed.