There’s no problem mixing disk speeds inside a stack. it use to be a problem with FC-AL which was loop based, not so much with SAS. So it comes down really to personal preference.
From the Storage subsystem tech FAQ
HOW MANY CROSSOVER POINTS BETWEEN SATA AND SAS CAN I IMPLEMENT IN MY DS4243
The term “crossover point” refers to the number of times a DS4243 storage shelf makes the transition from
SATA to SAS (or vice versa) within a single stack. Multiple crossover points per stack are supported.
NetApp recommends minimizing the number of crossover points when initially deploying a storage
configuration. As storage configurations grow, it is likely that additional crossover points will be introduced
into the storage configuration. This is acceptable; however, by reducing the number of crossover points in
your stacks, you reduce the variability of the storage configuration. As is true in all computer systems today,
increased variability can add difficulty when troubleshooting issues and also raises the possibility of
encountering unknown issues.