I use 'Platsil Gel 10' but many people use 'Dragon Skin' also. Dragon Skin is a bit less expensive but, I've heard that Platsil has a better tear strength.
Silicone is a dense solid rubber so, it doesn't 'expand' into the mold. But there is an expanding foam called 'Soma Foama' that's a silicone foam you can fill the puppet with if weight is a concern. Silicone puppets can be heavy because it's a solid rubber so, the foam is worth considering.
I use the both of them on my puppets, casting as skin in the mold and then filling it with the foam. Sometimes I apply the foam to the armature instead of pouring it into the mold because the foam rises really fast and it's hard to trust that you'll fill the mold properly when you're working that fast. (you also need to drill release vents into the mold if you cast the foam in it so it has somewhere to escape after completely filling the mold)
If you've already got your sculpture done, I'll need to ask what type of clay you used? That's because silicone will not cure properly against some materials. (like latex or molds made from clays containing sulfur) If your clay contains sulfur, you can still use the mold made from it but, it should be coated with a wax based mold release to insure there won't be a bad reaction.
I've never had that kind of reaction happen with my castings and I suspect that some of the warnings about bad reactions are somewhat exaggerated.
Also, what's your armature made of? Wire is easiest but, ball & socket is best - if you have a ball & socket armature however, just as with foam latex, you should wrap the joints in either food cling wrap or pipe tape to prevent them from getting filled with the silicone.
Hey Ron, speaking of drilling air holes for Soma Foama, off the top of your head, where would you suggest drilling them for a humanoid puppet, gypsum plaster mold that was originally designed for foam latex? (FYI- haven't cast any foam latex in it yet so no issue there). I'm thinking fingers, head, feet. Do you drill them all in such a way so they are sandwiched between the two mold halves, or any through the actual body of the mold? (I wouldnt think so...) Do you think the extra cavities for overflow foam latex would be a problem for the expanding foam? The overflow cavities come fairly close to the puppet cavity at some points and end up leaving a relatively thin wall of gypsum between the puppet and the cavity.
Lots of questions I know, thank you in advance!