> Is it possible for pups in a litter to have different sires?
Yes definately is. I had a border collie way back in the 1970's and I left her with a neighbour while I went on holiday. When I got back I spotted her loose in the garden of the neighbours house tied to a flatcoat dog! I was very cross as I had particularly asked my neighbour to keep her from male dogs. I told him I was now going to have to take her for the 'morning after jab' but he said there was no point as his spaniel pointer cross had mated her two or three days earlier.
Then I noticed she was coming out of season, so on consulting the vet was told it would be safer to let her have the pups. When they were born five were big black chunky puppies who looked exactly like the flatcoat and the other three were smaller and grey coloured with flecks of colur on their legs like the springer/pointer cross. I kept one of the flatcoat collie crosses, she was more like a flatcoat than a collie in every way.