Julie.
As to your questions, I think most here would only buy one rhizome of each cultivar. With time you will find that some do much better than others, but most irises multiply very well on their own. If they don't thrive and multiply, you probably don't want them anyway, so one rhizome is often enough. Obviously it will also depend on how patient you are.
If you want to prolong your season it could be a good idea to also try a few Standard Dwarf Bearded(SDB) and Intermediate Bearded(IB) irises. In many climates these are hardier too, it certainly is true here in Sweden, but please don't ask me how I know.
In particular they usually handle wet and cold winter weather better.
You may also enjoy looking at other iris vendors than Schreiner's, for instance Mid America Gardens:
http://www.beardedirisflowers....
I have not personally tried them, but for members of this forum they overall seem to be the most popular choice, so I don't think you could go wrong with them. There are many others vendors too.
If you haven't done so already, it is definitely worth checking out the vendor list at the top of the forum:
The thread "Vendor list" in
Irises forum
