The Eight/Charley stories are completely self-contained; the other Doctors appear in Zagreus
, but not in a way where having heard other stories with them is important.
The Six/Evelyn stories are less so as they go on; plot elements in the Seven/Hex tales prove quite important to the resolution of Evelyn's story. (Which, fact fans, is the only Big Finish to make me actually cry.) But you could successfully listen to just Six/Evelyn stories, it's just that you'd get less out of it all.