old cards shouldn't slow down your system, but they will use the pci bus less efficiently than the newer cards. depending on how you use your rig, it may or may not matter. i say may or may not to avoid an argument.  
 
 
no, i don't think you can go z-link to z-link betwen card and box or multiple boxes, but S/C would have to confim or deny that. you CAN use both z-link and adat i/o at the same time, however and there is talk of being able to link multiple boxes(in the distant future i'm sure) and i/o expansion(probably sooner).
btw-part of what has slowed down the production is S/C listening to the requests of users. the added mic pre was a serious job. Jurgen did considerable research before designing the pres and the HiZ inputs were added by request. the reason that there are 2 adats AND z-link was by request. the headphone jack was by request. Jurgen told me that he and Holger agonized over whether to make changes, but it's as Jurgen said, they have one shot to get this as close to right as possible and then the hardware is done. the software can always be added to, updated and changed, but the hardware gets designed and the vendors of parts gear up and there's no changing. Holger took notes and has a list of requests and things to check into, what a huge job! the good thing is, these guys are very happy to be doing it.