I have my Rebel XS, the kit (17-55mm F/3.5-5.6) lens, a 430 EX II with a small white card strapped vertically to the back of the flash to direct a dab of flash directly at the subject when I bounce off the ceiling, and a tripod. Shots will likely be indoors, after sunset, so no natural light to shoot with.....
You're nowhere near North America if it will be past sunset in 2 hours.
All the light you will be using inside in will be natural light - a photon, is a photon, is a photon. It doesn't matter if the photons comes from the Sun, an incandescent light bulb, or a camera mounted speedlight.
The only real difference is you will have a lot more control over your main light by using a speedlight.
Your main source of problems would be from using mixed light. Your speedlight will produce light at a color temperature very close to 5500° K.
If tungsten is the available light, they will be at a color temperature somewhere in the range of 2700° K to 3300° K (before lampshade color is considered).
If they are flourescent they will range from 3500° K to 5500° K.
Either the speedlight can be gelled to match the available light or the available light can be gelled to match the speedlight. Gelling the speedlight is usually easiest.
Are you planning on letting the camera make all the decisions by using TTL flash? Or were you going to handle exposure by using manual mode for both the camera and the speedlight?