If we can ignore the 'reusable' in the question: commercial launch providers publish a user guide that specifies everything you need to know to make the payload compatible with the launcher.
Compatibility consists of several areas, for example:
- the spacecraft has to physically fit inside the payload fairing and has to fit on a payload adapter.
- if the spacecraft needs communications, electric power etc. it has to fit the provisions made by the launch provider.
- the spacecraft has to withstand the launch environment (vibration and noise levels generated by the launcher).
In some cases, the launch provider can make changes to accommodate your spacecraft, e.g. a new payload fairing (larger or with specific connectors built in) or payload adapter. Changing the launch environment is much harder.
Launch profile changes depend on the final orbit you need, so every launch has a unique profile based on final destination and weight of the spacecraft.
The user guide indicates that the amount of propellant can be changed to suit the mission. I suspect this is only done for the liquid stages; solid stages will get a standard amount of propellant and burn time (if not, they'd have to do custom castings for each launch, which would be expensive).