Flashing (R530 onboard) Dell PERC H730 mini to IT mode

There's nothing in the brand names to stop you connecting them together. If you have the right interfaces (external SAS for the D2600 isn't it?) then you'll be just fine. I've got Dell servers plugged into HP storage, HP Servers plugged into Dell storage, all kinds of other things talking to each other.

The PERC 6/i is a dual-port controller; each port has 4 SAS lanes. On the 8x2.5in R710 chassis, that's a one-to-one mapping of front-panel disks to SAS lanes. On the 3.5in chassis, ports 6 and 7 are unused. With a 4-disk array, you could move 2 disks to slots 4 and 5 to split the workload between channels, although there's still the single processor and memory on the PERC card.

Updating firmware is typically a good idea, and is a fairly painless process (although it does require a reboot.)