May I connect 18W fluorescent tube to the electronic ballast from 17W energy saving lamp?
Yes, it is possible. The fluorescent lamp will probably light a little bit less but it will not be a visible difference.

Is it better to use the electronic ballast from the energy saving lamp than the regular one?
I don't think it's better to use the ballast from an energy saning lamp. It can be maybe cheaper way but more risky. I stopped to use them and replaced them with electronic ballasts for all fluorescent tubes I have. They are intended for use with the linear tubes. Ballast used from the enegry saving lamp isn't intended for that purpose and you never know when your flat can burn out. I used to use them with any problem long time but I wanted to have a calmer sleep :-)

Which wattage value of the energy saving lamp should I regard when I use its electronic ballast for lighting? There are often 2 vaues listed on the cover like 9W – 25W, 11W – 40W, 15W – 60W, 18W – 75W, 23W – 100W.
It is important to follow the real wattage of the energy saving lamp not that which is compared with the bulb wattage. So, for connecting the 25W linear tube you need the ballast from energy saving lamp which has listed the first value 25W.