If I am using 3/4" Hardwood (Acacia) flooring planks to put on the stair treads, do I need to leave an expansion gap. Should I glue and nail them down? or Just nail?

Thanks!!

The stair step is such a narrow width compared to a floor that expansion will never be an issue.Stair do get a lot of stress compared to a floor. You will need to secure all of the treads to the risers well with extra fasteners, even though they may have been glued together when built so there will not be squeaks in the future.
It may be a title messy to also glue and nail, but good insurance that the fasteners will never get loose. The fasteners only need to be relied on until the glue sets.

Even if stair treads were to expand they would take the path of least resistance, which would be toward the nosing and would not present any resistance. PL Premium Adhesive is good stuff for attaching stair treads.

Great info THanks! Another question, if im using my flooring boards for the treads, can they be installed directly to the stringers or do they need to have plywood underneath?

3/4" stair treads alone are too thin to span the stringers. Stair treads without subtreads are usually 1 1/4" thick. What are the existing stair treads? You need to keep the stair tread building code parameters in mind when you're doing your work. If you change the stair tread thickness you need to observe the stair riser code. The first step from the first floor landing to the first step and the top step to the second floor landing have to be taken into account as far as the building code is concerned.

I see. Its 3/4" plywood. The stairs are currently carpeted. Pretty tricky open ended stairs that are curved. I will be having a company that specializes in staircases come out to do it because I think this is beyond my capabilities. Thanks for all the great info!

FWIW, the correct thing to do would be to leave the 3/4" plywood in place. Per the building inspectors, the carpeting on the stairs would be considered to be the same thickness as the new treads so nothing would change as far as the riser heights are concerned. Hope you checked out the company you are bringing in.....not all worker bees do the same quality of work. The flooring forums have lots of horror stories about sub-standard work.