[Fofo]

I'd recommend reading up on quarantine procedure before getting more rats. You should definitely get Roxy another friend or two, but it's important to follow proper quarantine before doing introductions.
It's hard to say whether your rats who died had viral infections (which are contagious) or if it was just mycoplasma infection (not contagious). If they were kept outside they could have been exposed to wild rats carrying viruses.