I get my information from the games but this should have all the info.
http://combineoverwiki.net/wiki/Combine
English
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All I've been able to find is that they are a multiverse force that is attempting to take over the multiverse, not the omniverse, and not that they have come close too it as no one knows what actually motivates them. With the Time Lords we have a very real understanding of what they can do, and unless the Combine can survive having their universes destroyed, their timelines locked and their very existence deleted from history, there's nothing they would be able to do to stop the TL from taking them. The Combine would need absolute space-time mastery to even compete, not merely the ability to travel between universes. The Forerunners and Precursors could do that casually, yet they're no more capable of taking the Time Lords.
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I get the two mixed up. Doesn't really matters what drives them, like I siad I get my info from the games, it was stated by G-man that he wanted to prevent them from reaching their goal and it had to be done soon(which is to control the multiiverse). Also, thing is, any technology you use against the combine is reverse engineered and used against you. Time Lords would be fighting the Time Lords while the Combine sat back and watched.