Further, I don't worry about possible lead in the coloured bit, especially if you use the ones with the coloured part in the middle. I'm not sure I'd use the clear ones that have a coloured film coating on the outside.
Not only is it surprising how many you actually need to displace the air space, but there's also the hazard of getting them into the carboy/demi-john/jug/etc. These aren't known to be made from the toughest/strongest of types of glass. You could be a little rash in getting them in, to find that they've caused stressing on the fermenter glass and the additional weight causes enough stressing for the bottom of the ferment to crack/break off. A waste of good mead if you ask me......
I just make my batches a bit larger than target, in a plastic fermenter bucket, then once it's dropped a first sediment, rack to glass. Any left over goes into either a glass pint milk bottle (might be unavailable over your side) or a cleaned out pop/soda bottle. Any airspace in the pop/soda bottle can be removed by squeezing the bottle before capping. Then that goes into the fridge, so when I need some, I have a nice little reservoir of appropriate top up......