I don't know of any professional private guide, but if you'd like to consider part-time ones, let me know your budget and the number of hours and whatever requirements you have, and I have friends who are willing to earn some extra cash.
Hey, it depends on when they're there... I'll be happy to drive them around town and show them the sights. It may not be as detailed as a professional guide... but one of my first jobs in Singapore was that I was a tour rep working for my aunties tour company way back in the mid 70's.
Best to let the 'locals' or rather in my case, once a local, to show them round town.
Singapore is the place in Asia where you least need a tour guide. Cheap taxis that don't cheat you. English is as widely spoken as in Sweden, probably spoken by more people than in Sweden though you might have to listen carefully, lah. All the signs are in English. Lots of free tourist literature.