3rd party SSD with High Sierra
If I install High Sierra on a 3rd party SSD, will it be APFS?


An SSD is a drive regardless of who makes it and will be compatible APFS.

In this article, about SSD support for OWC (3rd party) SSDs, there are two main take aways:


OWC shows up as both a rotational drive, which won't convert upon setup and as a RAID 0 array which won't convert at all.
A user in the forum confirms that the Crucial M4 in a 2012 MBP works without issue.


So, the issue is (IMO) not a High Sierra one, but one of the manufacturers in how they implement their SSD and present it to the OS.