Is WD Green good for storing data?

I personally set 3 performance stages for my data:
- performance oriented (critical software like OS and generally used applications) - SSD is welcome here.
- storage/perf oriented (large applications which would likely waste SSD space, like games rarely played) - 1TB decent performance drive 7200 rpm, like WD Blue or Black
- storage oriented (movies, mp3's and other archives) - for this purpose you can use any drive, like WD Green.

This is my configuration, but generally if you plan it well any SSD+HDD configuration will do excellent job.
While i wouldn't recommend WD Green for an OS drive due to bit of lack in performance, if you use SSD for this purpose and WD Green as a secondary drive this will be cost effective balance of performance and large storage space.


 
Of course not, 99% of applications during installation asks for path to be installed, thus you can manage to choose which are considered important enough to be put on SSD.
Before SSD even appeared on the marked it was still valid and very good idea to separate installations over multiple partitions for a simple reason; it was far easier to manage OS partition with size like 100 GB considering defragmentation. Even more, it is known that the further from the first sector of the HDD the lower performance, so even back then the idea was valid, I just adopted it to current technology :)





 
It does have fragmentation in terms of location of the data, but its not important at all since access time to any block is really quick.
With typical magnetic HDD access time is determined by speed of mechanical parts like headers and rotational speed of the magnetic dish and that makes a whole difference. HDD header has to move from point A to point B, longer the distance longer it takes, The best is that all data in the reading sequence lay down on the same track so actually header doesn't have to move at all. So generally defragmentation makes that all small portions (blocks) of one file are keept as close to each other as possible so this reading sequence is optimal.

I'm not gonna mess your mind about that, most people doesn't run defragmenation anymore because it takes quite some time and at current technology of HDD's only enthusiast/professional users will notice the difference. But thats something to keep in mind when you feel HDD gets sluggish in reading data.

"SSD only have 1 type/size correct? (not sure if there is 2.5 and 3.5 like the normal spinning hard disk)"
Yes and no, most of them have typical 2.5" size, but remember there are models which can be connected to M.2 slot or PCIe so will look different.
2.5" size is most common since SSD's primarily was made to fit in laptops instead of typical HDD's, due to it's lower energy consumption...