How tall and broad are your Yews? I would think the large-leaved Rhodies would complement the Yews better than either the small-leaved rhodies or azaleas. Their ten year size would be of the order of 6-8 feet. Personally I would be tempted to take the Yews out.

All azaleas are rhododendrons and there are small leaved and large leafed rhododendrons. There's really no difference in ease of growth between azaleas and rhododendrons. The differences, rather, have to do with the specific varieties, especially in Z5. So, plant what appeals to you, but check with the nursery about your specific site and see what they suggest.

In Illinois you will be best off with cold hardy deciduous azaleas or cold hardy rhododendrons. Evergreen azaleas would take a while to get started. The young evergreen azaleas have not reached their full hardiness.

I am going to refer you to a Minnesota site and the plants they recommend. It is an excellent site. You are in the fringe as far as raising rhododendrons and azaleas, so you need ones that are hardy in your area. The ones on the Minnesota list should do very well for you. Good luck.

Minnesota Extension Service

University of Illinois Extension

Rhododendrons and Azaleas for the Midwest (ARS)

How to Grow Rhododendrons and Azaleas

Here is a link that might be useful: Azaleas and Rhododendrons for Minnesota

Thank you all for your help. I didn't realize that there were so many kinds, and that azaleas & rhod.'s are the same. Also thank you for the Minn. sites.