Low light and a modern camera? Start at ISO 400, 500,640,or 800. Those are the absolute minimum ISO starting points for me. ISO 100 is throwing away two to three stops of shutter speed or aperture; ISO 100 is great for perfectly-exposed, low-noise smeared, blurred images in "low light". With a shortish lens like 35mm, image magnification is not all that high, so the slowest shutter speed one can hand-hold with practice is likely to be as long as 1/8 second. However, at 1/8 second, anything that is moving, even slowly, is very likely to be a blurry smear; at ISO 100 a light level that requires 1/8 second will be shortened to 1/30 second at ISO 400, and 1/60 second at ISO 800; speeds of 1/30 and 1/60 second are much more likely to yield keeper rates in the 50% or better range, while at 1/8 second, keeper rates might be as dismal as 10 percent on many types of scenes and subject matter.
As far as shooting wide-open, I do not think that is a good idea at closer distances; depth of field is very shallow at f/1.8 at closer ranges, and often times there is so,so little depth of field that there's no room for error in focusing. At close distances, the focus at the edges of the frames is significantly different from the focus distance at the center of the frame, and the difference in actual, measured distance at the edges of the frame, as opposed to the center of the frame, will be enough that there's not enough depth of field to cover the difference between a center AF point and an off-center AF point, so focus and recompose will not work at close distances with the lens wide-open.
What you want is a little bit of what is called "a cushion" or "a safety margin"; the easiest way to get that cushion is to dial the ISO right up to a medium level, between 400 and 800. That will give you a cushion in terms of either shutter speed, or depth of field and focusing; if the subjects are moving a bit and shutter speed is the needed priority, you can slightly favor speed over f/stop; if you are working at close distances and depth of field from stopping down to f/2.8 or f/2.5 would give you a bit of a cushion, then stopping the lens down is *easily* accomplished by beginning at ISO 640 or 800 instead of ISO 100, in low light. If you have a modern, full-frame camera with superb high-ISO performance, it's practical to begin at ISO 1000,1250,or even 1600, or with the newest uber-cameras, even ISO 3200,especially if the low-light situation involves motion, like sports,dance,etc.