Anything added to liquid latex rubber (polyisoprene) will interfere with the elasticity of the final cast other than perhaps pulverized rubber particles. Unless you need strength and stiffness in the final item, you trade off the ability to elongate and return to the original shape.
There are numerous neutral fillers used for all sorts of material casting, anything that is water soluble can very likely be added to liquid latex. Anything that is particulate matter will cause the latex to shrink around it, causing it to become harder and stretch much less. Acrylic paint added will tint latex, tremendously reduce flexibility, and make it much harder, more adhesive - making that it will be hard to demold without using a separator or painting in an intial layer of latex without any additives.
When making a half mask with my high school students who aren't worried about archival quality length of life of the piece, I've experimented with a variety of things. After painting in an initial brush coat, I've had students follow with torn up pieces of paper towel dipped in latex, much like you would do with Papier-Mâché. They may apply a second Stuck later and a final brush in layer of plain latex.
For purposely hard casts like stunt props, I've used multiple layers of paper or torn pieces of cotton fabric, like old bedsheets. You can experiment with the latex you normally use in a small mold or just cast a chin area within your mold to see what results you get. Back in the days when I mass produced masks they were pure isoprene, other than some pigment.
Mold making latex has fillers already added for bulk and is flexible but stretches less. This requires less layers but you won't save much on cost. You can buy it in arts and craft stores in small containers but when you calculate the cost to acquire a gallon it's less expensive to buy a good mask making latex like Mabry or RD-407, which is faithful recreation of the original Cementex product.