Most of the tour starts from Urumqi then to Kashgar. It is ok to start from Kashgar instead?

Urumqi can be easily accessed from other parts of CN, that is the reason why most tours start from there. But it is certainly ok if you reverse it. There are still some trains from Kashgar to Urumqi for your choice.
The train tickets should be in great demand on 30 Sept. It is highly recommended you book the tickets in advance. Maybe you can turn to TravelChinaGuide for help. They can book the tickets for you. I have got their e-mail address: bookingtravelchinaguide.com. May help!