There are of course descendants of Portuguese, Spanish, English, Dutch and French settlers here. The Portuguese were first, and they had a village in Russei Keo for centuries. By the mid 50s Portuguese names were still common in the neighborhood but the people had interbred and assimilated to such a degree to be indistinguishable from Khmers. I worked in that neighborhood way back and met people who claimed to have Portuguese ancestry but they had Khmer names by then. There is a street near the Chruoy Changva bridge named after a Portuguese Col de Monteiro, who was King Norodom's secretary.
As for the French, the first settlers were nearly exclusively male, and they did indeed often cohabit with local women. As the protectorate developed, this practice was discouraged and French officers often had French wives with them. There is a good book called "Bad Frenchmen" by Gregor Muller which covers this.
As with the Portuguese many mixed French-Khmers would have eventually become indistinguishable from the local people. Many would have left after independence and before the wars, or tried to conceal their past, those who remained may have been targeted during the Pol Pot regime.