There is a on going research/breeding project on Echinacea at the Chicago Botanical gardens. I would contact the or Dr Ault for more specific info about cross pollination and isolation distances (i dont have that kind of specific info on ornamentals).

http://www.chicagobotanic.org/research/breeding/echinacea_article.html

best wishes
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I guess there will be any cross pollination.
I did some of those crossings at mine place and the result was true seed.
As far as i know the Echinacea purpurea is self incompatible, and the seeds i harvest was on the Ech. purp. after a cross with pallida and paradoxa.

What are you planning to do with the seeds you harvest.
I might be interested for some of it.
In exchange i have seeds of most Echinacea for you.

ps. Excuse me if my language is not correct. coming from Netherlands, Europe