Actually, if we're talking about the V3's, the 12 in the Rumble 100 is Ceramic, not Neo. I like the way that cab sounds with a little extra TLC, but they aren't doing it as an extension cab. Nor do they do a 210 extension cab, which would be the perfect compliment to the Rumble 500.
Pairing any decent 8 ohm extension cab with the Rumble 500 is going to help it out, on three different fronts. First, the total wattage jumps from 350 to 500 watts. Second, the extra speaker(s) in the extension cab will help the stack to move more air, watt-for-watt. Last but not least, the two internal 10's that were working overtime to handle 350 watts, will have a little more headroom, since they will only be shouldering 250 watts out of the total 500 watt output.
Like Linnin says, out of the two cabs offered in the V3 line, the 115 is the logical mate to the Rumble 500. It's similar in size and weight. It handles 300 watts continuous, which means it should handle the 250w per side that the 500 watt head can push into a 4 ohm load. It should be closely matched to the 2 10's in the internal cab efficiency-wise too, so both cabs ought to be pulling their weight. Each of the three woofers would hit their limits at about the same wattage and loudness, so cone breakup, compression and overall tone ought to blend about as well as a mismatched pair can.
The 410 would work as an extension as well. It's 23" wide and 14.5" deep, just like the 115, so stacking with the 500 combo wouldn't be a problem. The 410 would be loafing with only 250 watts to drive it, which would be good for longevity. Also, it would move more air than the 115, watt for watt, so the larger stack would play a little louder.
Still, the big stack seems like overkill. The 410 is much heavier and more expensive than the 115. With only 500 watts to share between the internal and extension cabs, the 410 would never reach even 50% of it's 1000w program/500 watt continuous rating. It would stand a little taller, but the extra 4 or 5 inches would barely be noticeable.
Once again, as Linnin said, the 410 is a more logical pairing with the Rumble 500 head, since that still leaves you with the logical option of adding a second 410 for a 4-ohm, 500-watt 8*10 stack.