Sit in a chair, if you extended your coccyx the requisite amount for a tail, and made the muscle base strong enough to support the whipping motion needed for a good strike you couldn't sit very easily, you'd need to sidesaddle pretty much every chair.
As for how the muscles attach, look at something that uses its tail as a weapon (and doesn't have an exoskeleton, scorpions don't count because their musculature is screwed to hell and can't be used in a mammal.) think iguana. The second thing is to think about your balance, tails play an important factor with that, and you don't have a lifetime of use in one to compensate. You'll be a newborn kitten testing it's claws for the first time.
The spine is important because of the sheer, mindblowing pain that comes out of spinal injuries that don't cripple. It'll be sensitive, and vulnerable to breaks.
How much coordination does it take to land a snap punch on a moving target? Adding new limbs means you have to learn how to do that kind of thing all over again, with muscles you've never used before in your life, and your species hasn't had in any form for many millennia.
New limbs aren't impossible, but they aren't the best use of the Mutagen.
What, like a xenomorph tail? Anyway, as for subdermal shielding. What we need to power it and how would that work? It sounds like a good idea.
Most implants require you to have an internal capacitor installed, that itself can be recharged with standard energy cells over time. As you live and breathe the capacitor will also gather a portion of your waste energy, very slowly recharging on its own.