I would not introduce new fish to a recently problematic tank for about a month, I had issues in the biggest of my 6 tanks mid summer and it went into complete lockdown until very recently, the new fish could infect the recently sick fish with something else or the healthy new fish could catch something from your recently sick Molly.
Regardless, it is far safer to quarantine new fish, again for several weeks. You are only likely to take this approach after having a tank wipeout shortly after adding a new fish directly to an existing community, believe me it does nothing for your fishkeeping confidence and you can lose your favourite fish in the whole mess (I lost Steatocranus casuarius "dad" amongst others after adding 4 Upside Down Catfish directly to my existing group). I bought a lovely 3rd Synodontis brichardi last Friday, he is in quarantine in my 620T, showing no signs of illness and his thin body is already filling out with no competition for food after normal small portions in a fish shop tank. If all goes well I have 4 African riverine species to pick up this Friday, they will go in what will then be a vacant Rio240 for a quarantine period, although I am prepared to seperate them out further if any of the groups need special attention.