[H] NAS storage

[Winterlight]

I am planning a Synology DS918+ NAS DiskStation, Diskless, 4-bay; 4GB 
DDR3L purchase. I have 3 WD RED 3TB drives so I am planning on buying 
another one for 12TB. They all HAVE to be the same size ...right?

90 percent of my storage needs is for my  video collection .. around 
3-4TB plus my video ISO collection = 1.5 TB for a total of 5 TB of 
available storage. I do not need to clone these drives in a RAID as I 
have both hard and off site digital backups of them and they do not 
change that quickly. I don't want to consume my entire 12TB of 
storage for video. So what would be the best way to handle this.. put 
the video and video ISO collection on a USB3 external drive and plug 
that into the NAS which would make it available to my single user 
LAN? That would leave me with a great deal of storage space for 
computer backups, security camera, Data that needs to be secure= 
about 15 GB, media center recordings, stuff like that?

Or is there another way to do this without having to clone all the 
video files all the time? I am confused.
Thanks.