There may be a risk of reversion but I doubt that would have anything to do with crossing with the green. Variegation is just not a completely stable trait. I don't know how unstable it is in Pachysandra. I certainly wouldn't worry. What's the worst that can happen? You have the silver for a few years and then you have green. If you don't buy the silver you don't even have a few years.

Pachysandra 'Silver Edge' is a popular groundcover selection here. But I have never seen it revert to any significant degree so that would not be an issue I'd be overly concerned about.

It does spread more slowly/less rampantly than the solid green, so there is a possibility of the green crowding out the variegated but I wouldn't even consider that much of a risk either.