CQ, a couple of points. Previous posters such as Daisy have correctly pointed out that your own dogs can (and arguably should) be trained to have a routine when the door is knocked and/or visitors come to the house. But in your case, this is not just about your dogs - you regularly have fosters in the house too and from what you say, were going to fetch some rescue puppies as well.
Because you are a foster home, you should therefore have a backup set of management techniques which EVERYONE in the household should be trained in and adhere to regarding things like household security, door opening routines etc. In fact, I would expect the rescue you foster for to go through this with you, since by their very nature a foster dog can be an unknown quantity, and is not yours, thus requiring extra levels of care and precaution.
I would put in place the training issues already discussed, but I would also (if you continue to foster) adopt a clear, easy routine for everyone in the house to follow when the door is knocked and visitors arrive. We have a similar routine as our front door opens on to a tiny front garden and then the street, and although it's a very quiet residential area, there are quite a few cats around - not a good mix with sighthounds. So, we have a system whereby whoever opens the door is responsible for ensuring that all dogs are safely out of the way. This allows me to sign for parcels, fight off religious callers or chat to my neighbour in peace.
There is also an issue of reporting the incident to the rescue you foster for - although you've said your friend is currently not minded to call AC or take legal action, you might not have been so lucky, and there could be complex issues of liability. I would discuss this with the foster co-ordinator and suggest that lessons are learned. If not already done so, foster carers should be reminded of this issue, and requested to put in place appropriate routines to prevent this happening again.
Lastly, and i realise this is not your responsibility, but I'd be very concerned about friends bringing dogs round when you had potentially new rescue pups. When collecting relinquished dogs, it is very common for them to be unvaccinated, unwormed, and to be carrying fleas, mites and various other parasites/illnesses. I'd want to minimise their exposure to other dogs until I was absolutely happy about their status, and I'd also want to protect any visiting dogs so again, I'd instigate a routine of being very clear with visitors about when it's a good time to bring their dogs. As with many doggy issues, it's more about training owners than it is about the dogs. 
Hope you are starting to feel a bit better now.