I would like to know if its possible to buy the special "Shoshazan Ropeway" combination ticket, but instead of getting on the bus at Himeji Station, I want to get on the bus from Himeji Castle?
Yes that is possible, you can ask the information center the exact bus stop number where you can take the bus to Mt. Shosha. The stop is located where a bunch of restaurants are located in front of Himeji Castle.
What we did was take the sightseeing bus (which was around Y50-150 I think) and then got off at Himeji Castle. Did the Castle and Koko-en Garden in the morning and then ate at one of the restaurants located in front of the bus stop where we had to take the bus to Mt. Shosha.
When we got to Mt. Shosha, we chose to walk instead of taking the microbus, you'll see a bunch of statue on the walkway up to where the temple ground is. It's quite tiring for someone that isn't fit however it was a great experience seeing all those statues and looking down where the farms are. Also note how long it takes you to go up and just estimate how long you'll need to go down because if I remember correctly, the last ropeway is at 4:30 or 5:00 pm (we went in November).
I think doing the Mt. Shosha first is more of a hassle (if you're allowed to get off between mt. shosha and himeji station to get to the castle) because after you do the Castle, you would either have to walk or take a bus to get back to Himeji Station and I'm assuming that it'll be dark by the time you finish the castle.