Hi, how wonderful that you help by fostering

You know on re:reading your post, you say that Perdi does not play with toys, so is her play and interactions with other dogs, running, chasing and tumbling with them? This could be why she only plays with
selective dogs.
Young SBT's play quite roughly, compared to other breeds, although JRT's are very hardy they are still small, she probably chooses who is suitable to play with i.e the older and calmer foster dogs, along with the fact he is 'new' I dare say he is pretty boisterous and she isn't keen.
However, he is very excited to see his own kind by the sound of it, .........all you can do is distract him from her, be his playmate, try to get him to focus on you and allow him near Perdi when he is only calm to sit with her and sleep next to her when you are all snuggled in the room together, creating a nice social bond, the sooner he accepts you as the playmate the better.
I don't know how long you'll have him, but with any other dog, I'd be playing training games with a whistle or clicker to focus him and obviously teach him commands, soon take his mind off Perdi.
Granted it's harder with a foster dog, maybe in another week things will calm a little.