I would avoid using the inbuilt flash if at all possible. Especially if you plan to produce mono shots - because using a high ISO sensitivity instead of inbuilt flash will generally look nicer in terms of lighting and mono conversions of noisy shots often look quite good because in B&W, noise looks a bit like fast film grain sometimes.
Also, shooting indoors in mixed lighting can be a nightmare for white balance. Although it's usually best to try to get the best WB setting before you convert to mono, the conversion will disguise many white balance problems... another reason that your preference for mono might be a good idea.
If you're going to be doing lots of indoor candids or informal portraits in future and this isn't just a one-off, it's worth considering an investment in an external flash. Bouncing off a ceiling (if it's white or neutral in colour) or using a diffuser like the various Gary Fong gadgets, often gets
much better results than direct flash, and best of all it's relatively easy to get big improvements in your shots this way.
Unless light is good, I would stick to the wider of your lenses and get in closer to your subjects, because you might find camera shake a problem with the longer focal lengths.
I'm assuming you probably won't get opportunity to use a monopod or tripod, because manoeuvring around a room full of family jollity is hard enough when your camera is handheld, in my experience.
