Mixing HDDs for different purposes?

[Enquiries]

Brilliant guidance across the subject - thanks.
Following it, I'm upgrading my Synology DS212 (which was struggling) to a DS920 Plus with 2 Western Digital Red Plus 6TB EFZX CMR drives. In part, this is to incorporate some future proofing.
My main use is as a Media Server. However, at some point I may consider adding Surveillance functionality. Is it possible to populate one of the spare bays with a suitable (e.g. WD Purple) HDD and dedicate it, on its own, to that role? If so how do you go about it and does the new HDD need matching in any way to the specification of the others? If possible please can you give an indication of what sort of capacity such a HDD should have in light of it overwriting earlier footage.
This subject could be of wider interest for those wanting to have a similar mixed functionality.

[ed]

Thank you for your support, this keeps us going.You have made an excellent choice. DS920+ is their latest multimedia friendly NAS model. Thanks to a dual LAN you can indeed use one of the bays for a dedicated Surveillance solution. You will be allowed to create a separate RAID (Basic mode with single drive) and with a separate LAN connection, your main RAID/Volume will not suffer from a slowdown. If you merged all drives together then surveillance constant writing would keep those drives bussy.You may need 1TB storage space for recording 30days of surveillance in 1080p 15fps. You can play with a calculator here: https://www.synology.com/en-uk/support/nvr_selector?cameraNumber=1&storeDays=30&perdayhour=24&hdmi=0&codec=H.264&resolution=1920&fps=15&mbps=3.25Better resolutions will mean more storage space required.DS920+ is a future proof model with an option to increase RAM and add an expansion box. It is also not limited - you can install any app from their package list.I hope this helps.