If you are only fostering piggies I would not house them with your own, if your existing pair get on well then a third temporary female is added this will change the group dynamic and a new pecking order will be established then when you remove the foster piggy the pair *** have to sort out their pecking order again.
A bit stressful for your original pair, also there is the issue of quarantine, if the rescue has quarantined and treated the piggy for mites etc and kept it for some time to be sure it is free of fungal or other problems it should be fine health wise mixing with other piggies but it would be worth checking how long the sow has been in rescue before you put it with your other two just incase it is carrying some hidden problem. Often with a move and stress hidden ailments manifest.
This all depends on whether you are ging to be a 'failed foster' and end up keeping the rescue sow or if you are going to be able to part with her.
Personally I would not mix another sow into an existing pair unless they were going to remain as a trio.
Good luck with your fostering.