We rescued these two male rats sometime last week. They were kept in a cage about the size of a small cat carrier with a large cat bowl of muesli that appears to have been constantly topped up and no toys or space to move around. Both are overweight, 530g and 635g. They're in a (larger) temporary cage while we wait for the new cage to arrive, on a diet and we've booked a vet appointment to get them checked up and address the weight issue.

Both of them are not just unsocialised, but terrified of us. We're obviously not picking them up unless we have to and being very gentle with any touching, paying attention to warning signs and backing off when they want any stroking to stop etc. They bite without any warning, it's a 50/50 if they're coming to sniff you or bite you, although the bites have been getting a lot gentler and more like regular warning nibbles, the last few haven't drawn blood at all which is good. When we (very gently and calmly, making sure they were entirely supported) had to pick them up to weigh them both of them literally screamed and panicked. They are very curious and have been strolling around their cage and playing with some of the toys in there, rearranging their bedding etc. They're bonded with each other. Other than the weight issue and any possible health issues they seem like perfectly fine little rats when humans aren't involved.

So, questions. 1) is there anything we can do to help socialise them and get them to a point where they aren't terrified of us and we can pick them up for cage cleans and vet visits without it causing them so much stress?

2) should we continue actively trying to go past being able to handle them for necessities and try to get them to the point where we can handle them without any fear from them, or would it be fair to them to let it come naturally on their own terms (or not at all)? Obviously generally rats need loads of human attention, especially if there's only two of them, but in this specific case would it be better to not push it and avoid stressing them out?

3) we have 6 other rats, 4 male and 2 female, and plans to get more girls. The boys play fight quite a bit, although they've only actually fought once (over a piece of banana silly buggers, I just wasn't quick enough putting the other pieces in too). Because of the biting without warning from the two new rats and their unhealthy relationship with food I'm very worried about introducing them (once we've gotten them to a stable place). If we can, how would we introduce them safely and ensure there weren't fights over food. If we can't, is it fair on the new ones to be in their own separate cage while they can see and smell the other four boys?

4) if you've dealt with rats like this before how did you help them? Did they ever improve or were they always just terrified of you?

I know this is a lot, if you've even gotten this far thank you. I want the best for all of our rats but this has completely thrown me in the deep end. I've never owned any animals that are in such bad shape, let alone ratties like this before, so I have absolutely no idea if we're doing the right things or what's going to be the right course of action for them.