Actually the capacity of the depot doesn't need to be modded. It is SERVICE capacity, so that's the amount of the vehicles that can be serviced at a time. It doesn't mean any limit to the amount of vehicles stored there actually.
The thing with that though is that cities the size of the ones in CiM2 would have bus depots with high capacities. If you have a depot with 100 buses, around 85-90% of them will be out on the road during daylight hours. The rest will be either sitting unused awaiting peak times and/or be in the workshops being repaired.
At night though, you'd probably find that around 60-70 percent of that 100 will be in the depot as night buses would use less vehicles than daytime routes. The only way to stop buses queueing in the game is to have unrealistic frequencies on the night buses which means lots of buses running around empty wasting money.
If anything, it would be good if you could "add" a depot next to an existing depot in the game where it would become one larger depot instead of a seperate depot with it's own lines.