Most often, silicone doesn't adhere to other materials, except to some ceramics like glass or wall tiles. Hence the suggestion of metal or plastic as a mold. Liners are seldom necessary but exist if needed.
Users sometimes want silicone for medical cleanness or biocompatibility. Such a need would impose the choice of one material.
Yes, silicones are extremely varied, not only in hardness. I've forgotten a bit, but there are several families (3?) depending on the way they cure: by evaporation of acetic acid (not good within a closed mold), addition of a cross-linker (may include tin salts), and one (?) more. That would be the first decision, because some manufacturers or distributors provide only one family. Then, each family provides a broad choice of hardness.
Molding is rather easy, the biggest worry being bubbles, because of viscosity. It's a good idea to obtain a vacuum pump and bell jar, transparent for observation, to degas the resin after mixing has brought air bubbles in. Casting under vacuum would help, so the mold is properly filled. For bigger series, people mix the resin directly under vacuum.
Shape accuracy isn't a worry. I had poured (tin-linked) resin in a Tupperware: every so tiny scratch in the mold was visible at the part.