Yes, it is compatible with most Traxxas Slash models, including the T-Case and Turbo.
This motor is designed for use in the AXIAL HSP HPI trucks, but may not work as well or last as long as it would on a Traxxas Slash truck. However, if you have an AXIAL HSP and want to try this motor out, it should work fine.
No, this is a dedicated 12T brushed motor designed specifically for the ARRMA trucks. It will not work on your existing HPI ball motors.