Yes, pretty much needs to be on all sides to be the most effective. The top gantry beam could probably benefit a great deal by using that method. You should be able to mount the rails right on the fiberglass surface if it is flat. You can also drill through it into the concrete if needed. I'm not sure how you intended to mount the rails. Were you planning on embedding steel pieces in the concrete to mount the rails on. You can drill and sand the fiberglass outer layer just like anything else. It should be easier to sand than concrete. If you mount the rails first in the DWH putty you could run the fiberglass right up to the rails afterward just leaving enough for clearance, but it is not as strong as having a continuous covering.
As far as rebar sizes. I always thought the thickest you can afford is better. But I am not so sure of that. Usually, there is a rebar budget and it is placed in the areas of stress where it will do the most good. You can have too much rebar but I think with your design more would be better than not much because those beams although very thick by steel standards are not very thick by cement standards. I'm just guessing but I think it would be better to have four half inch pieces of rebar in those side beams than 1 piece of 1 inch. And half inch rebar is easier to bend around curves.
Have you given any thought to how you are going to cast the pieces and assembly them together? Or were you planning to make one monolithic casting?
Paul