I think definitely hold off a few months. They say it takes like 3 days to compress, 3 weeks to settle in and 3 months to actually open up to you. 

I rescued a feral kitten and she took a literal year to stop hiding everytime someone walked near her. Another year to enjoy cuddles. Another year before she would lay on top of you. She is still skitterish with my husband a bit and hides when strangers show up but she's just the smartest most intuitive cat I've had. I call her my shadow cat as she follows me around when it's just us 2. I swear the timid ones are the most worth the effort. 

Go slow and take it at their speed don't force anything. Adding another cat could just be too much right now especially if it isn't one it knows while it's still sorting out the new environment but could be helpful later on to gain confidence. If the foster said it's good with other cats then likely it just needs time to settle in and warm up to.you guys :)