Re: The Five...err Four Doctors!
Certainly, Sarah was originally with 4, so I'd guess any serious thought of using Jo came in the brief gap between losing Jamie and losing Tom: the Brig moves from 3 to 2, leaving 3 in need of a companion until Sarah becomes available to pair with him.
"Some days are better than others. They say that where I come from."
"Loudly, I imagine, on the day you left."
(Blake's 7 - Rumours of Death)