Yes, it is possible to use the Synology DS923+ NAS initially with only two hard drives and later upgrade to four hard drives.

### Capacity expansion
- The Synology DS923+ supports up to four hard drives directly in the device and can be expanded by an additional five bays with the DX517 expansion unit, allowing a total of up to nine hard drives.

### Different hard drive sizes
- You can use different sizes of hard drives, as the DS923+ supports both 2.5-inch and 3.5-inch SATA HDDs and SSDs. However, it is advisable to use hard drives with similar storage capacities and speeds for optimal performance and data integrity, especially if you are using RAID configurations such as RAID 5 or RAID 6.