You could use that as a former, anything really but if your going to use it for a beam the former might as well have some structural integrity. Aluminium can be quite heavy for its strength when it is formed into a tube and you will loose its benefits when its wrapped in Carbon. I've used balsa but found I had to use so much Carbon it was quite heavy and also expensive due to the amount of carbon I used. Balsa laminated was better. The best so far has been Canadian Spruce, same quality that is used in aircraft spars. Its light, cheaper then the aluminium/balsa I was using and very nice stuff to work with. I've made some Laser Tillers using Spruce wrapped in carbon that have stood up to years of punishment, none have broken. You could build a box section spar from spruce then wrap it in carbon.
I'm sure there are even better products to use as a former out there somewhere, I'm always on the look out.
I wouldn't worry about vacuum bagging until you have built something that might do the job, once you have the processes involved in building sorted out and a prototype that has been tested and looks like it will stand up to the job you can then build the final piece with the object of maximum strength and least weight. Testing it against the prototype will give you confidence when its used in the final application.
I've done alternate layers of carbon wrapped at various angles, seems to work out well, its going to be very messy though.
You need to keep in mind that the info I'm giving you is the result of experimentation in my spare time, I like to tinker, see what I can get away with, I dont get to concerned if something fails as I will just build a better, stronger one.

Work out what you are aiming for by testing what is in place now.

Darryn
Mosquito 1704
South Australia