...and remember, too, that while Oakville and Burlington "grew" from their original cores, Mississauga, in 1968, was essentially a new entity created from suburbanizing Toronto Township. It had no core to speak of--Port Credit and Streetsville were (then) separate municipalities, and acted more like independent enclaves a la pre-1966 Mimico and Weston. (Closest thing to a makeshift "core" was Cooksville.)
And re "periods of development", remember that Burlington and Oakville were quite tentative at allowing its sprawl to leapfrog the QEW before the 70s--but in Toronto Township, it was already quite rampant by the 60s, thanks to developers like Shipp and the overall commuter-friendly proximity to Toronto. So you already had the "sprawl in search of a centre" problem...