Can expand different storage HDDs in 1 Backplane?

I have Dell Server R740. This server running with 1.2TB 10K RPM SAS 12Gbps x 6qty (RAID-5). Now storage is not enough. So, we need to expand 2TB x 2 qty storage.

My question is below. Please kindly give suggesstion. Thanks.

Can expand different storage HDDs in 1 Backplane?

e.g. existing HDDS : 1.2TB x 6qty

       new      HDDS : 2TB x2qty

Can expand with different speed?

e.g. existing : 1.2TB 10K RPM SAS 12Gbps

       new      :  2TB 7.2K RPM SATA 6Gbps