You already have two parttiem clerics, so you don't really need another half druid. But between Minsc, Anomen and yourself, you should have plenty of fighters as well. Yoshimo will do as a thief, although I personally prefer to take Jan. Taking Jan instead would also mean that you have all the bases covered even without Jaheira or another 6th partymember. But, since Yoshi's such a good friend of yours, by all means keep him. You're a little low on mages perhaps, but if cose combat and archery is more your style than magic, you'll manage until you can rescue Imoen.
So, you'd have to fill the slot with another tank. Mazzy wouldn't fit in because she's too short. Valygar is a ranger, so with you and Minsc in the party that would make three. Since rangers are such a cool class, I'd say go for it, but not everyone may agree (My first party, BTW, was a ranger, with Minsc and Valygar, Yoshi/Imoen, Jaheira and Aerie. That worked well enough for me, and as it turned out I never had to kick anyone out of the party. I hate kicking people out. Anyway, just wanted to say the two look very similar).
Alternatively, you could pick Keldorn. So, just run the checklist.
*Can the problem be solved using different weapons
-yes: solve it
-no: go on to the next question
*Are you romancing Jaheira
-yes: keep her
-no next q
*Is it really a problem or are you just imagining it?
-imagining it: keep Jaheira
-it's a problem: dump her
You've now determined whether or not to keep Jaheira in your party. Now, there's an open slot
*Do you know which NPC, if any, should fill it?
-yes: fill it with the NPC you had in mind
-no: next q
*Should it be anything other than a tank?
-yes: Pick Jan
-no: next
*Do you mind having one shorter character in the party?
-no: Take Mazzy
-yes: next
*Do you mind having 3 rangers in your party?
-no: Take Valygar
-yes: Take Keldorn
End of problem
