Pretty much as the title says, I have an 8gb pi with 7.4gb spare memory but as it has to be divided by 3 I'm wondering if I can use the SD card which has 53gb spare under /storage
Kodi will cache to /storage/.kodi/temp *BUT* unless your network is broken Kodi does not need to deviate from default settings; and if the network *is* broken the correct solution is fixing the network, not crippling Kodi with weird cache changes.