The BlackHawk Shot Shell Slings will not work with a FABARM STF12 unless you have a rear sling attachment point on the stock.
The BlackHawk Shot Shell Slings attach to the shotgun using sling swivel studs. If you Mossberg Maverick Security has sling swivel studs then the BlackHawk Shot Shell Sling will work. If it does not have sling swivel studs it will not work with that shotgun.
The BlackHawk Shot Shell Sling uses (2) HK style clash hooks to attach to a firearm/shotgun. The buttstock of the KSG will be able to equip one of those hooks, while the front will need a picatinny sling mount in order to attach the other hook. I would recommend the FAB Defense SLA Sling Picatinny Attachment which will mount to any piece of picatinny rail located on the forend of your shotgun, and allow you to attach the other hook of this sling. You can find the picatinny sling mount in the link below:
https://www.opticsplanet.com/fab-defense-sla-sling-picatinny-attachment.html
Yes, this should hold 20 gauge shells as well as 12 gauge.
This sling is made with 2" wide nylon webbing. The Magpul swivels are made to work with 1.25" slings, and will not be compatible with this sling.
This sling will work with a Mossberg 88 that has standard sling swivels.
Yes, it should work as long as your Shockwave has the sling swivel on the bottom of the pistol grip.