When you hire a nurse, can you immediately assign them with the same action?
No. Hire is one action and says that the nurse goes to the parking lot. Assign is another action.
Is a nurse considered a bonus in terms of being able to return a cube after healing?
Also, if using multiple bonuses, can you use both without pulling the full amount of cubes needed? For example, if you have a doctor with +2 in a ward, and then have an additional training token to give that doctor +1, and lets say you drew 6 cubes and got what you needed, but want to be able to return some cubes to help future healing attempts, do you have to pull another 3 cubes to be able to return 2 cubes?
You do not have to take a bonus. If you take a bonus, you must take it in full. You can return only cubes of bonuses you actually took.