Okay, so I just came back from my trip with my family to Singapore and Malaysia yesterday.
I exclusively used Scapia for all the Grab taxi bookings I made and any other forex purchases I needed to do. I also had an ICICI forex card and Axis Atlas (rarely used it) as backup.
Mainly used the MRT and buses for point to point travel. Both Scapia and forex card worked fine without the unnecessary headache of forex markup.
If you don't have elderly people or infants in your family, I would suggest using the public transport system in Singapore as its top notch and saves you a lot of money in the long run.
Airport rides will exclusively be in Grab assuming lots of luggage.