It is not uncommon to use smaller speakers for the surround/rear channels. For most applications, there is typically less content in surround channels than in the Left, Center, and Right channels. They typically demand less from the monitor, and to save on cost, you may decide you do not need as large of a speaker.
It is fully possible and technically supported by the GLM software to combine any SAM capable monitors to the same system. It is quite typical to have the surround channels with smaller models and the main LCR with something bigger. The only important thing to remember is that bigger monitors always have more power and dynamics available, so when combining them with smaller models, the limiting performance in the system is with the smallest model in it. Mainly this is the issue with maximum sound pressure level.
Please also note:
Can I mix Classic 8000 Series monitors with SAM™ Systems?
If you have any questions regarding monitor choice, please contact our Genelec Support Team.