i can definetly say that that method will work, but your mold wil deteriorate much faster than using silicone made for moulding.
a normal mold would last you around 60 copies
this wil maybe make 5 before major tears and cracks will show up.
but hey, you can always make a new one.
also, try and make a hard shell around your silicone mould with resin and fiberglass, this will keep your mold in place when you make casts, so no warping or dents occur. there should be enough tutorials on the great internets.
(source: own experience and we own a shop that makes moulds on a daily basis )