Guilin - Guangzhou and Guilin -Shenzhen is a cheap flight line in China. Better to check how cheap will be the air tickets before you look into softsleeper. Re travelling by train in generall, as Guilin - Guangzhou/Shenzhen is not a hot line, so it'll be easy to get softsleeper although act as early as possible is still a must.
Soft sleeper price for your reference: Guilin - Guangzhou from 383, Guilin- Shenzhen from 444.
You can see what still available real time on this site launched by China Raiyway Ministry: http://www.12306.cn/mormhweb/kyfw/ypcx/ . Just select date and paste departure and arrival city names( in Chinese Guilin=桂林, Guangzhou=广州, Shenzhen=深圳), put in security code then click 查询, you will see how many and which kinds of tickets are still available in seconds. I‘d like to give you some meaning of Chinese characters to help you understand well:
车次 Train No.
发时 depart time
到时 stop time
商务座 forget it (very few, only available in bullet trains )
特等座 forget it (very few, only available in bullet trains )
一等座 1st class seat (only available in bullet trains)
二等座 2nd class seat (only available in bullet trains)
高级软卧 luxury soft sleeper (a kind of private carbin with 2 beds and 1 toilet in it, it’s the best choice for night travel, but costs a lot even more than that of flight.)
软卧 normal soft sleeper
硬卧 hard sleeper
软座 soft seat
硬座 hard seat
无座 no seat, sit on your legs.