How can I know what rhodies to buy so all will bloom at the same time?
The blooming time is a genetic trait so the safest way to get them so they will bloom at almost the same time is to buy plants of the same variety that originated from the same mother plant (even then the weather can affect some of the plants). The second choice would be to buy plants of the same variety regardless of the mother plant. The third choice, when adding different varieties to the mix, I would add varieties from the same wholesaler if the wholesaler describes the blooming period by month instead of using a phrase like early, mid or late(you may have to call them to get this info). This is useful because the various R. austrinum -for example- varieties bloom anywhere from March 30 through April 20. Again, do not forget that Mother Nature will also affect these dates, sometimes quite a lot. Feel free to discuss this requirement in detail with a master gardener when you call local plant nurseries.
PS - Since the blooming period of rhodies and azaleas can be short lived (about 2 weeks more or less), you may want to use plants that bloom at different times so you always have something blooming in Spring.
Luis, thank you so much for your kind response, and yes, to consider different varieties in order to have on going color is a great consideration. Thanks again. Hay.