There are some requirements for skeleton hierarchy: legs and spine should be attached to the same pelvis/hips bone (sometimes they’re in a parallel hierarchy). Root bone on the ground under pelvis is recommended. Orientation of joints co-directed with bones (not like Daz3D skeletons). But within this restrictions all models should work.
You can switch meshes using the same skeleton on runtime just using SkeletalMeshComponent → SetSkeletalMesh. But if they had a different size, it would worsen calibration.
It depends on attachment points of trackers. Arm Span is measured as a distance between trackers attached to hands. You can add some adjustment to get a correct calibration with your positioning: go to BP_CapturePawn2D → CaptureDevice component → Details tab → ArmsCalibrationAdjustment (in centimeters).
Just like p.3, but need to increase ElbowLoweringMltiplier.
P. S. I don’t check this topic often. Please write me at e-mail for support (about custom skeleton, this situation needs investigation).