The couplers/sleeves should be as tight a fit as possible, so the "bending" forces are distributed on the full length and not only on the edge at the end and the struts are kept as straight as possible. Shimming will help, but introduces point stresses wich is also a problem.
Couplers that give both inner and outer support to the struts/poles used with a nice snug fit would be the strongest. I bet you could even get away with machining them out of a stiff plastic if you make them long enough and you kept the core support as a solid piece.
I'm really excited about tensegrity stands and hope to experiment a little myself with some discarded Laser top-masts I think I may have laying around somewhere. They are aluminum, but may be too bendy, we'll see.
Thanks for the inspiration and good reading in this thread everyone!