
Hi Lauren.
From reading your post, I would suggest leaving Cassie as she is. i.e. a single dog.
And I tend to agree with what a few others have said too - she may need a little longer just to get back into her 'rhythm' with being in a new house etc.
I owned my two Staffords for 10 years +, and although they got on very well, they were always separated when no-one was at home. Which would defeat the object in your case, of providing company for Cassie.
Another dog could cause more harm than good possibly, and upset the balance you're already getting.
My male Stafford has been without our female Stafford now for 3 years (we sadly lost her to cancer). He is a single dog again. He's of a similar age to your Cassie. And he's absolutely fine with his own company. Quite happy to mooch around during the day and nap, and then whizz about and play games with us in the evening.
(I should add - I work from my home office, so I am with him during the day. But that has not always been the case, and he's just as happy whether I'm at home or not).
As a side note - I was very unsure how he would behave with another dog in our home. Our son arrived home just two weeks ago for a visit with his 12 week old German Shepherd pup. Chaos ensued! lol In a good way. They got on great. But I would still not leave them unattended. Staffords are Staffords.
I hope Cassie settles and you find some harmony :)