Q1: As for mixing the enclosures, I believe this is yes according to both the quickspecs and cabling guide.
P2000 G3 Mixed-connect expansion configurations:
The following illustrations show examples of expanding storage from P2000 G3 array enclosures to multiple drive enclosures of different models. Adding a P2000/D2700 mixed-connect configuration The following figure shows a dual-controller P2000 G3 MSA System (1) connected to a P2000 6Gb drive enclosure (2) and D2700 6Gb drive enclosure (3). Data throughput in the array performs at the 6Gbps rate, as both the controller enclosure and the drive enclosures support 6Gbps speeds (assuming that all disk drives in the array are also 6Gb drives).
Q2: I would say no, according to the SMU Guide:
http://h20564.www2.hpe.com/hpsc/doc/public/display?docId=emr_na-c02520791
A vdisk is a “virtual” disk that is composed of one or more disks, and has the combined capacity of those disks. The number of disks that a vdisk can contain is determined by its RAID level. All disks in a vdisk must be the same type (SAS or SATA, small or large form-factor). A maximum of 16 vdisks per controller can exist.
A vdisk is a “virtual” disk that is composed of one or more disks, and has the combined capacity of those disks. The number of disks that a vdisk can contain is determined by its RAID level. A vdisk can contain different models of disks, and disks with different capacities. For example, a vdisk can include a 500-GB disk and a 750-GB disk. If you mix disks with different capacities, the smallest disk determines the logical capacity of all other disks in the vdisk, regardless of RAID level. For example, if a RAID-0 vdisk contains one 500-GB disk and four 750-GB disks, the capacity of the vdisk is equivalent to approximately five 500-GB disks. To maximize capacity, use disks of similar size. For greatest reliability, use disks of the same size and rotational speed.
Q3: According to the cabling guide it should operate at 6GB backend rates.
Q4: You can use "straight-through" cabling with the p2000 which may provide performance benefits, with a tradeoff of redundancy. So I would recommend to follow the P2000 cabling guide and other P2000 supporting documentation. Same link referenced in Q1.
Hope this helps to answer your questions.