Activated carbon is very abrasive - you should be cautious about using activated carbon. We are currently having some toothpastes tested for both abrasive and whitening. We should have the results back in a couple of weeks.
However, you do need to consider why the hydrated silica is there - is it an abrasive grade or is it perhaps a viscosity modifier. If you are removing it as an abrasive, you may not need to replace it. If it is used as a viscosity modifier, then you need to consider your options carefully.