I prefer a single mesh, but if the costume or accessories will change, it is better as a separate object.
In your scuba tank case, I would have a specific bone for the tank and weight all the verts to that bone. Then use some helper bones and constraints to keep it in place (w/o twisting or deforming etc), but still retain the ability to animate it as well (because you might need to manually fix it for some poses)
It’s a tricky thing to explain how to do via text but that is the general concept.
As for armour and clothing, it’s always a pain the butt to rig. But don’t be afraid to hide the mesh underneath when the character is wearing a costume, and if you look at game models you will actually see all the armour sections deforming in the tricky spots anyway 😉
