I have the curly rush in my bog, bare root, in pea gravel. I think it will grow as long as it is in the water. The way I do it, if I have several plants, I put them in different locations as far as sun, depth of water, flow of water, etc. and see which does best. Sometimes the movement of a waterfall or stream is best, others don't like the moving water, but want deeper planting depth, etc.
If you fertilize, be sure to use something that will stay in the gravel or dirt, whatever you have your iris in. You can use pond plant tablets, or another cheaper way is to use granular with the same numbers (i.e. 8-16-8 or whatever your plant needs). Wrap the granular stuff in a piece of paper towel, then shove it down into the pot or gravel. The paper towel will dissolve and allow the fertilizer to slow seep out to the plant. I know pond lilies are usually pretty heavy feeders, needing fertilizer once a month during the growing and blooming months, not sure about the iris. I'm sure it won't hurt!