Drobo v1 drive question/suggestion

My Drobo version 1 is still humming along w/4 2TB drives and dashboard 2.8.1 (I also have a 5n). I’m thinking of swapping in 4 or 5TB drives, one at a time, and other then them being SATA drives, does the rotational speed need to match up to what is already installed? For sale I see 5400rpm and 7200rpm, and most at 6 Gb/s. Any suggestions welcome.

The individual drive speed should not matter.

I believe that a Drobo Gen 1 (USB only) can not support any HD larger than 2 TB. If if does, I would be interested to know about that since I have my Drobo Gen 1 still in every day use.

Drobo Gen 2 (Firewire), I have successfully used a 4 and 5 TB HD, also in every day use.