Chosen Solution

Planning to buy the mini with flash storage, apple customization page says it is PCIe based flash drive, will it be possible to add more internal storage using a 1/2 gb 2.5 inch Hard Drive?

Yes, the HD connection is SATA III (6.0 Gb/s). Right now the needed hardware and ribbon cable is not known, but someone has used the one of the older mini cables and has it working (so he claims) we have yet to verify with some else nor get the part info for the cable.