space between new plants

I have just started a bit of gardening in our back garden. I've lived here for a few years but not much gardening has been done for many years. We want to replace horrible weeds Ground Elder and dug them out as well as the old soil (and stones!), added compost and planted a Salvia (amethyst) and some Campanulas. My question is how much space do I need to leave between them, although I've planted them yesterday. I started reading afterwards about the spread and that they could spread up to 0.5 metre. Would it cause a problem if I planted the campanula too close to the Salvia? It looks nice as it is.