this can be done, i explain how here , but its a bit ‘challenging’ if your not used to linux 
hopefully C&G will release an image where this is already done…
(we hoped to do this after OS 3.1 was released)
anyway for now, you can also use a USB hub to allow for more devices.
as for longer samples, basically it all comes down to what you want to do with them
recording/playing can be done direct to/from sdcard/usb of long lengths.
its limitations are you can only play/record forward in realtime e.g. no running backwards, editing, to do more than this you have to bring into memory.
also there will also be a bit of latency in triggering a sample when its coming from disk (should not be too bad though)
I will say many of these are just limitations of the externals used for direct disk playback, there is not really a technical reason that much more cannot be done off disk. but that needs a bit of C++/file system knowledge to implement.
(really surprised someone hasn’t done this for PD, is really not that hard)