Crazychem. has covered your questions pretty well.
I always thought the BIC Rhumba had a centerboard..... does yours?
Next question is do you use the centerboard very much?
Nothing to be afraid of here, but if you use the CB alot, you will have some adjustments in technique to make if you get a smaller short board without a centerboard.
Are you trying to plane through your jibes with the CB down? That basically won't work, for a number of reasons.
What is your weight? Your weight will have a lot of impact on how small a shortboard is most appropriate for you.
Something in the 110-140 liter range would seem good, but again, if you are heavier, you may want something bigger and wider to take advantage of the early planing characteristics.
As far as your rigs go, if the 6.4 m2 rig is the one with the green/aqua or reddish pinkish colored mast, then it's the original rig that came as the package with the BIC Rumba.
Yes, it's very heavy, due to the very heavy mast. The sail is not too bad, and if you put it on a high carbon 460 cm IMCS 24-26 mast it makes a pretty good rig.
You will also need a new mast base as the BIC setup left alot to be desired.
Whether you decide on a GO, a Carve/Futura, or something else is going to be based on what's available, what you can afford, and your conditions.
Sounds like you have the conditions to make a 6.4/7.5 rig size quiver work pretty well on a 120 liter board.
Hope this helps,