Welcome Tamara! That should be a very interesting bed when they are all in bloom. I always start my plants about 18 inches apart, and they seem to fill the spaces quite fast. You can plant them 15 inches apart, but you would need to thin them quite often to keep them from running together. I'm not sure if you have all of the dots both grey and black set at 15 inches apart or if just the Black dots are 15 inches apart. I wouldn't add anything besides the irises in the spaces as they seem to like the air flow around them, and they sure grow fast enough to fill the spaces quickly.
One thing to remember is that all irises are not created equally, and some will grow much faster and fill the spaces more rapidly, others will be slower to grow. Some will grow taller and some will be shorter, so you might want to check the average height of each one you chose. The data base on here will give you that height, also many vendors list the heights of their plants.
Any way you look at it, remember gardening is a journey, not a destination, so it's fun to try things and learn from them and make adjustments as you go along. What ever you do, it will be a learning experience for you, and I can't wait for you to show us the pictures of your project as it unfolds, and for sure when it's all in bloom!
Good luck, I'm sure others will add some ideas and suggestions too.
