If you want or need to travel soft sleeper and end up with no women in the compartment, usually it's pretty easy to get assistance from the attendant and switch with somebody. Especially if the train isn't booked solid, which it probably wouldn't be this time of year. Don't sweat this too much.

Itinerary: If you have a month or nearly so, you can easily cover this and then some more. For this particular time of year, this is what I recommend:
Beijing (4 nights/5 days). Overnight fast train to Shanghai (2 nights/2 days). Day train to Suzhou (2 nights/2 days incl overland daytrip or 1/2 day to water town). Fly Shanghai (SHA) to Xi'an, or if flight available from Wuxi airport to Xi'an, that's closer than SHA. Xi'an--2 nights/1.5 days. Overnight train to Chengdu--2 nights maybe a 3rd if you want to daytrip to Leshan Buddha. Overnight southbound train to Panzhihua junction then change to bus for Lijiang in Yunnan province (4 nights). Road or flight to Kunming (2 nights), if you want to stop in Dali, go by road and add 2 nights there also. Fly to Guilin, overland transfer to Yangshuo (4 nights). Fly Guilin-HKG (2-3 nights). This adds up to about a month. I'd spend any additional time you might have in the relatively warmer parts of the south (lowland areas) according to your interests.

If you have the cash and prefer to fly, most of these segments are directly flyable by nonstops. Chengdu-Lijiang nonstops are more limited in number, but if unavailable on days you wish to travel, you can also fly Chengdu-Lijiang via transfer in Kunming, where there are more plentiful connections.