Drivers in the small sub-depots do get paid less because of the lack of over time they get.As for buses:
I am sure that if you went round every Ulsterbus depot in Northern Ireland I am sure that you would find examples of the same buses operating the same routes every day.
You did say that Coleraine does not follow this rule but you do often see certain buses been used for long periods on certain routes. Coleraine's Leopards tend to be swapped around between different routes alot while the newer buses would tend to stick to the same routes. Coleraine's sud-depots would always use the same bus on a route. I think the drivers like it this way but it can provide a problem for a CL driver who is covering for some one in the sub depot.
On the other hand many drivers don't care about what bus they use. I know of one driver for example who always takes the bus which is closest to him when he arrives at work each day!!!
On my old school run we always got Leopards such as 112 (ROI 112), 173 (VOI 173) or 281 (AXI 281) for periods of about 2 weeks at a time, so it kept you wondering every day what bus would you get.
When talking about buses allocated to the same routes, it is always interesting to see which bus covers for it, when it is in the garage etc. In CL one think I have often noted, is that when 1273 (OXI 1273) (a Kilrea bus) is in being serviced, sister bus 1272 (OXI 1272) is often given to Kilrea - is this to try and keep things as normal as possible?