RN316 - confirm mixing disk sizes

All these disks will work.  You could also use the equivalent Seagate disks.  You will get a warning with the 7200 rpm models, basically telling you that the performance will be limited by the slower disks.  Based on my own experience, that can be ignored.  7200 RPM drives will use more power (and therefore run at higher temps).  I've chosen to stick with the 5400 class myself.

Note that many larger disks don't have the side mount holes (because they want to maximize room for the platters).  So with some of these models you will need to remove the plastic inserts, and use the screws that came with your NAS to attach them to the tray bottom.  The holes in your tray bottoms won't all align, but even one pair of screws is enough to hold the disks securely.  If you've lost those screws, you can purchase them from third parties - let us know if you need a link.

FWIW, I had a mix of WD60EFRX and WD100EFAX in my RN526x for a while, and had no issues.  The WD100EFAX does have the side mount holes.  Note the newer WD101EFAX is air-filled - it will use more power than the older WD100EFAX, and from what I've read is a bit noisier.

Personally I'd get a pair of disks up front, and take some advantage of the increased space.  Bigger disks are generally more cost effective when expanding vertically (at least over the long haul).  Though you do need to factor in how much you are prepared to spend now.

In your case, just getting a 6 TB replacement would cost about $166 US (Seagate ST6000VN0033).  Avoid the WD60EFAX by the way - it is an SMR drive.  Anyway, $166 is about the minimum you need to spend.

• Two WD80EFAX would cost $426 (current USA amazon pricing), and gain you 2 TB of space - $130 per TB gained over simply replacing the 6 TB drive.  The formula is (426-166)/2.  Each 8 TB upgrade after that costs  $107/TB.
• Two WD100EFAX would cost $526 and gain you 4 TB of space which is $90 per TB gained - (526-166)/4.  Each upgrade after that costs $66/TB
• Two WD120EFAX would cost $700 and gain you 6 TB of space - about $89 per TB gained.  Each upgrade after that costs $60/TB.  
• Two WD140EFAX would cost $917 and gain you 8 TB of space - about $93 per TB gained.  Each upgrade after that costs $57/TB

Based on this, upgrading to 8 TB isn't that cost effective.  All sizes over 10 TB are about the same as far as cost effectiveness goes.  If you don't need a lot more space right away, then maybe go with 10 TB (since it is the least expensive up front). 

About a year ago I went through this analysis when I needed to replace two WD60EFRX - and went with the 10 TB option based on the pricing at the time.