Yes, or you can let them run free. They will stop when they reach the first of the dropper shot that you have on your rig (assuming you're using them for pole rigs).
Depends on what sort you buy mate, some of them are fixed in position with a plastic pin similar to a feeder stop and this particular type are ideal for avon float set ups in the bigger sizes. Others can be set in position with a shot either side, with pole setups its not essential to stabilise it but its advisable with running float rigs like avons.
I have the drennan inline olivettes which just have soft rubber either end so I will put a very small shot say number 13 where I want it on the line. Thanks for help.
I have converted to them big style for heavy stick and avon float fishing but have never needed any additional method of securing apart from the tubing at each end. I have found them to be superior to shot due to their neatness and because they do not nip the line as large shot would. I now use the Maver type, this is because they have a groove machined along them for the line to lay in, instead of it bending round the profile.
I stopped using the ones with the central holes as I was not happy with pushing something alongside the line to 'nip' it into position as once again the line can be distorted.