Yes - you can use all the different lead time options together. The plugin contains advanced logic to ensure that if more than one lead time applies, then it will always show the most appropriate lead time. For example:
If a global lead time is set, then this will only be used for categories and products that do not have their own lead times.
The category lead time will override the global lead time, and will only appear on products that do not have individual lead times.
A product lead time overrides the global and category lead times.
An individual variation lead time will override all the above.