My best advice is to make several varieties and see what works for you; most of the poi I've made have been with materials I've already had, or occasionally I'll have spent $2-3 for something that would make it better, but experiment with a lot of different things.
Heavier heads are easier for isolations. Stretchiness make isolations easier. Light heads hurt much less when you hit yourself. Non-stretchy cord will get you more ready for fire (if that is a goal for you).
I can't count the number of poi I've experimented with trying find exactly what works for me, though by now I've limited that to only two or three different sets, but fiddle around with weights, lengths (when I first started I did *not* like long chains, but I've since made much longer ones), handles, and materials.
But like Newgabe said, the best thing is to just try 'em out for a while. If eventually one of the balls flies away whilst you spin, you'll know there's a design flaw.

Give each one a good sharp tug to make sure they'll hold. As to the stiffness of the cord, I can't imagine it would be a problem, but you may find it uncomfortable as you practice certain moves.