Your mechanic is full of it; you don't need to replace your crank pulley to use underdrive pulleys. I've seen over a dozen different types of engines with underdrive pulleys on the stock crank pulley and it worked fine.
Now; if your stock crank pulley is in bad shape (as they can get to be after 10+ years), and the rubber is getting brittle or the two halves are beginning to lose their adhesion, which can cause a serious problem down the road, THEN you would want to get a new crank pulley.
There's nothing wrong with not wanting another OEM unit, though it would work perfectly well.
**This damper will work just fine with your other pulleys.**
It's worth noting that if you have a 4 valve engine (you didn't specify what you have, so I don't know), this damper will further underdrive your accessories by another 3%. I didn't see how much the SR pulleys underdrive the accessories, but I'd guess around 10%; that's a pretty standard number.
With the additional underdriving going on, you might notice some charging issues at idle - lights dimming, electronics acting funky; the alternator just isn't spinning fast enough to be able to charge the battery. In addition, the water pump may have a harder time keeping the engine cool at idle. Therefore, you may want to raise the idle up about 200rpm to compensate for it if you have the ability to do so - i.e. tuner, etc.