Easton makes arrow shafts by wrapping carbon fiber around an aluminum tube. What you suggest can certainly work but it isn't commonly done because of several issues. Putting carbon fiber in your truss structure will increase the cost substantially because carbon fiber is relatively expensive and the manufacturing process will be expensive due to labor or capital investment. An all carbon fiber structure will be much lighter than a hybrid using steel which is very heavy. Once you increase cost so much you may as well go all the way. Also bonding between the composite and steel will be critical to the two materials working together and sharing load. The thermal expansions of the two materials are different and most high-performance composites are heat cured and shrink on cure, so internal stresses can cause significant strength reduction. Theoretically a hybrid is a good idea. The complexity of making it work is pretty high.

In additon to Compositepro's response, most fibre in direct contact with metals cause increased corrosion, so you must also have a isolation material between.

Nick,

Is this a structure you want to develop/manufacture in-house? Or, are you considering outside assistance.

We are in the "discovery" stage of the project right now.  In terms of design, we should be able to apply general principles of composite design.  We would likely seek expert advice when it comes to finalizing the design.
  
It seems at this point and time that we would need to outsource manufacturing for this project.  My company is more into welding steel.  We have only recently released a small project which requires welding aluminum.  I am looking long term with the composite material.

You should look at Strongwell (formerly MMFG) which can produce pultruded I-Beams up to 24".  If I recall correctly they developed the large sizes for an NIST grant for offshore floating platforms where corrosion and weight were critical.

Maybe you could get some strength and $ information on beams that are exclusively composite.  You may find that the logistics and disadvantages of a steel / composite beam vs. just a composite may have diminising returns.