The Dominion were the colonizers. They very likely had changelings installed in key positions on every planet in their domain. Theirs was a policy of expansionism cloaked under a guise of isolationism - this gave them the rhetorical narrative of victimhood, while in practice, exercising the very real imperialistic expansion throughout maximum range of space.
The DQ-AQ wormhole just opened up an unexploited pocket within grasp of their fleet supply stations. Once the wormhole was discovered, either through the actions of aliens, or perhaps their own young seedlings, they would have exploited every corner to their fullest capabilities. They are not referred to as the Dominion by bitter adversaries; they refer to themselves as the Dominion - a word defined as "dominance or power through (legal) authority," which in sci fi terms could be updated as "enforceable authority".
Odo would have inadvertantly started the war sooner or later.
Now as for the Bajorans colonizing - it was an empty, ostensibly unclaimed, undiscovered planet. Is that imperialism? I doubt it; because colonists in these episodes tend to be small, independent groups of explorers and squatters - and not official expeditions bent on territorialization and exploitation - characteristics rightly levied at the Dominion, in spite of their careful cultivation of plausible deniability.
Planets that want nothing more than to be left alone don't send imperial fleets to take over a region of independent worlds. The whole "Solid vs Shifter" thing was just to facilitate the objectification of their acquisitions. For all we know their history was entirely fabrication - even more likely when you consider the possibility they had once been solids themselves! It wasn't the Solid they had an issue with. It was Raw Power, pure and simple. They just needed clear divisions, and rallying the Shifters was really all they needed, regardless of what shape everyone else was.
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