Are junctions allowed in an active panel?
Yes, you can have splices in the service panel. However, you cannot combine circuits that are required to be separate (if any).  You also cannot combine circuits, such that you're overloading the circuit.  

A breaker terminal (or any terminal really) can only have one conductor terminated, unless the terminal is listed and labeled otherwise. Some breakers are listed and labeled for two (2) copper conductors, but not all are.  If the breakers you're using are not, you'll have to splice a pigtail to the breaker terminal  (though based on your question, it sounds like that's what you're planning anyway).

Fill rules apply, but it would take quite a bit to overfill a panel.