Doing a Silicone Mold of a helmet won't ruin the original, that's the point of using Silicone, it doesn't stick to anything but other Silicone and other things, like the original sculpt or the cast agent, not really stick to it. You brush on the Silicone, build up your layers, then peel it back and pop out the original.
Making a mold and producing copies of the Halo 3 Legendary Edition Helmet itself is going to get you into a Morally Gray area. Typically making a mold and producing Copies of another person's work with out their permission is called "ReCasting" and it is a major No-No. Where this issue becomes gray is that making a mold of Fan's work, another Fan Sculpted or Fan produced item or helmet, is absolutely unacceptable, however, some people think that recasting a commercial product is acceptable, because it was a commercial product, and others still feel it is wrong because you are still making a copy of another artist's work, even if they were working for a corporate entity.