I am planning to make a homemade fixture to run This bulb. The bulb is 9W PL preheat (with starter inside) and originally was installed on a fixture with another same bulb (2x9W). And i Want to use a ballast that can hold 2x9W bulbs. And the question is: Can i use a 2x9W ballast to run only ONE 9W bulb? And This setup is bad for the bulb/ballast? I have previously used the ballast with only one bulb to test only and It worked well but if i keep It running a few minutes whan can happen?

Try and check the lamp current. If in the 0.15..0.17A range (the rated current for these lamps), it is OK.
The main problem with an elevated current would be the ballast overheating, so you have plenty of time to do the experiment without risk of harming anything.

Normally it should work, the current may differ by a few mA, but if tge ballast is on the edge of saturation, the difference could become greater.

Thanks i Will try when I find one of the two ballasts that I have stored. I have previously tested this setup with one tube and worked well and the ballast temperature not rised too much.

 Here all u need is a 8 watt preheat choke  to run this lamp

 There are some PL adaptors  out there thats screws into your socket that runs 5, 7, 9 watts  PL bulbs that u shown me in your photo.

Most of these ballasts are compatible with the whole range, from 1x F4T5 through 1x F13T5, 2x F4T5, up to the 2x PLS-9W (and all wattages between, just some may be limited on the bottom end by their saturation and that is easy to determine by just checking the current in the desired combination.