You have to be guided by the amount of ammonia or nitrite, you can't change a given volume. The water changes need to be big enough and often enough to keep them both at zero. The filter bacteria won't be harmed by water changes unless you forget to add dechlorinator. The filter bacteria live in the biofilm which is bound to surfaces so changing the water doesn't remove any bacteria.
When you say Tap Safe, do you mean a product made by Interpet, Bioactive Tap Safe? According to Interpet's website, this removes chlorine and heavy metals, contains aloe vera (which is unnecessary) and "beneficial bacteria to help maintain a healthy balance in your aquarium" which is mainly hype. It does nothing for water hardness.
If you have soft water, the mollies will not stay happy for very long. They need hard water. The best thing you can do is to return them to the shop and do a fishless cycle - see the link in the post higher up. Then choose fish that like soft water. To see just how soft your water is, look on your water company's website, it should be there somewhere. If it is United Utilities just type your postcode in
here
The best site for researching fish is
Seriously Fish. Their profiles tell you what hardness and pH a fish needs, what size tank they need and what temperature they need.
And to warn you - don't believe anything a fish shop says till have have researched it for yourself. There are good shop workers but they are few and far between. The rest will tell you any rubbish to make a sale.
