If I install High Sierra on a 3rd party SSD, will the main volume's file system be converted to APFS?
The hardware is a Early 2011 13" Macbook Pro with a Crucial M4 SSD.
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.
It's not clear from your question whether you want it to be APFS or not, but not all 3rd party SSDs have been compatible with macOS High Sierra's installer. Apple is aware of this and is working to address this in future updates, but it's unclear at this stage which SSDs will be supported or not.
However, if you're hoping that your SSD will work with APFS then your best bet is to just try installing macOS High Sierra. If the installer detects a problem with the APFS conversion it will cancel the install.
Finally, if you want to install macOS High Sierra and the installer doesn't allow this, then many users have found they can still install macOS High Sierra through a rather long-winded process.
NOTE: I strongly recommend you have a reliable backup of your data before commencing the following process!
Once you've finished you can reboot into macOS High Sierra from your SSD.