If I might make a couple of suggestions...
Don't bother switching ard P and M modes, just go to M and use the meter there. Take test shots and adjust -- interesting night lighting is often contrasty and can fool the meter.
For the flash exposure you can go manual too, if your flash allows it - allows for consistent repeated shots, even when you change angles etc.
Being lazy, here's what I would do:
Switch to Av mode (this ensures metering for ambient light even with a flash, for Canons. For other brands, you need to select slow-sync or similar.) Set to 2nd curtain sync.
Mount camera on tripod (handhold if you want a 'special effect' that could actually be quite nice). Set aperture. Check metering. Adjust aperture and/or ISO to get desired shutter speed, within reason). Take shot. Adjust exp comp and/or FEC accordingly.
You might even have a 'night portrait' scene mode that does the trick.
basically there are 2 exposures to consider
1. the "correct" aperture to use for the flash exposure
2. the shutter speed to use with the answer of no. 1 for the "correct" exposure for the background
set the camera up on a tripod
position your subjects
set your flash to rear curtain sync
use manual exposure and set to the answers that you got form 1 and 2
ISO speed don't really need to be high
Honestly I dont think flash is really necessary. I think prime lenses like the 50mm 1.4 or 1.8 would do the trick especially for low light.
Even better, since you're using the D7k, it has a pretty good ISO performance. So, you should really try to play around with the settings, control exposure etc. + singapore being a bright place at night, I dont think it would be much of a problem.
I was inspired by that light-painting that I tried on some friends. It was 2.5 to 4 sec exposure n they drew a heart (2ppl) using the phone's LED flashlight. The flash was on 2nd curtain n abt aperture f8 or f10. I realized that any body movement whatsoever before the 2nd curtain flash was not included. The result amazed everyone! You guys should try too, coz it's easy n I'm no pro either Cheers!
(Note: I had to try about 10+ times to get it right, hehe!)
Life is full of partings - snapshot them while you can & cherish them!