I think mixing it with topsoil may be a good idea since bagged garden soil sometimes holds too much moisture. You have been having lots f rain, so I would definitely avoid mulch and such that would keep the ground too wet. With bearded iris it is better to be a bit dry and avoid getting your fertilizer of choice on the rhizomes. With Japanese keep the soil wet and fertilize and mulch. Species iris are tricky because they range from true water iris to those that like the garden soils. I would do research on each kind to see what will keep them most happy.
