They are no too heavy, over a kilogram though. I would say it's more down to your positioning on the ceiling and how safe any fixing would be. Probably no heavier than the average light fitting. You would have to use the same size screw head as for the wall fittings.
The brackets have a keyhole design, so as they're going on the ceiling I would add a washer larger than the keyhole opening, just to be on the safe side. A little more fiddly to fit as you'll have to put the screws in after the bracket is held in place.
You don't have to of course, you could just make sure the screws are nipped up quite tight so the bracket can't move. But I'd add washers as I'm a bit OTT with fixings