NAS upgrade?

[Enquiries]

Hi, I'm very much an amateur at this NAS thing but love the potential and I want to learn more. Your website and videos have been instrumental! I've been using a synology DS216+II with 2 6TB HDD in RAID1 configuration and have run out of room and my system is slower than I want so I decided to make a big upgrade to the synology DS1621+. My question is about setting up the new system once it arrives. Do all of my new drives also need to be 6TB or can I populate it with 4TB HDD? I was thinking of getting four of the 4TB HDD that have a drive speed of 7200rpm and I was hoping that I could somehow transfer the data to those in either a RAID 5 or SHR configuration (more likely would do SHR if that is possible) and possibly get rid of the older/slower 6TB drives. Is this even possible? Or do I need to keep the older 6TB drives in the mix (I will have 6 bays so I guess I could...is there a downside to that?). l was hoping not to have to spend the extra money in buying all 6TB drives but will if I have no option.
Thanks! And pardon my ignorance.

[ed]

With DS1621+ you can mix different size drives or even create separate RAIDs/Volumes. In a home environment mixing drives will give you the best storage space. You can then later replace smaller drives with bigger ones to gain some extra space. If you used this NAS for VMs, web server, and other IOPS demanding things then I would recommend using same kind of drives. But from home and RAID5 stick with regular WD Red/ Ironwolf drives.I hope this helps.