Could I revert system to Yosemite by using TimeMachine backup, after upgrading system to El Capitan?
Yes, you can easily go back to a previous version of os x if you have correctly saved that version using time machine. 

You'll need to : 


connect the time machine to your mac
Restart your mac and hold Command + R
After the mac boots go to OS X Utilities and select Restore from a Time Machine Backup then hit next or continue
A screen will show Select a backup source select your time machine bacup
You will have next a screen with OS X Version and Backup Date and Time, If you want to go back to Yosemite select 10.10.X (Mavericks is 10.9.X)