25 ft. 8/2 Stranded Romex SIMpull CU NM-B W/G Wire

Pre 1996 when neutrals were bonded to appliance chassis and houses had 3 prong receptacles, yes. Post 1996 where the NEC requires separate neutrals and grounds all the way back to the main service and 4 prong receptacles, no.

6000watts / 240vac = 25amps. So yes, the 8ga. wire would have way more than enough capacity. 10 gauge would be a better choice for purchase as it is good for 30amps.. Hope this helped.

It is not safe and against codes.

NO! NM cable is meant only for permanent installations where the wire can be protected. You need an SOOW cable.

No

Ground is solid copper other wires black and white are stranded copper and typically do not need anti-ox

8/2wg 50a

Our electrician bought and installed this because we previously had a 2-pole connection but our new tankless water heater required 3-pole. It runs from our attic down to the breaker panel in the garage. Just what we needed.

Number 6 wire will b fine.

8 gauge is okay for your needs. One insulated conductor should go to the new hot buss bar in the new panel. The other should go to your neutral buss bar, whether or not it's mounted directly to the panel chassis or is stand-off insulated. The bare conductor is your chassis ground wire, and should be tied into whatever buss bar is available that's attached to the metal panel can. If the neutral buss bar is tied physically to the metal panel can, you can use it for the chassis ground wire also. Otherwise, you must tie the chassis ground wire directly to the metal can wherever it's possible, and make sure the metal can is properly and solidly grounded. If not, yes you can drive a copper grounding rod into the ground and attach it to the panel can. Check Code for minimum ground rod length.