OP I worked as a family lawyer for many years. Disclaimer that I don’t know your full situation etc but the risky father scenario is one I came across so very often.
Firstly, yes they can split up your newborn from their 15 year old sibling. All things being equal they would keep siblings together but they will not do so if it poses a risk to one of the children (this is assuming your partner even gets his 15 year old living with him).
Secondly, you need to leave your partner and you need to do it now. For the time being it doesn’t matter if social services are right or wrong in asking you to do so. Even if they are wrong to do so, the thing to do is leave now so you can keep your child (hopefully) and then reconcile if you manage to persuade them (or a court) he is no risk.
The longer you stay with him, the less chance of you keeping your baby. If you decide last minute to leave him, it may be too little too late as they may not trust that you will remain separated. You need to leave now and prove you can maintain the separation, as it is common for people to leave and then reconcile a week or two later. You need time to prove it is final.
The longer you stay with him the more basis social services have for saying you don’t take their advice, you don’t have insight into the risk he poses to your baby etc
They can’t just remove your child willy-nilly of course - they need a court order and we can’t possibly comment on if they would get it or not as we don’t have the info. But courts do frequently order removal of newborns on the basis of the father posing a risk.
I’m sorry that this sounds bleak as I know this is not easy, but please please don’t jeopardise losing your baby for the sake of a relationship. For a newborn, adoption could well be on the cards if they find you can’t care for the baby. I’m not meaning to scare you but you need to understand the stakes are so so high. Please leave.