And nobody ascends to the centre seat of a Romulan warbird by being stupid,dullards don't tend to last long.
Perhaps Tomalek's ambition simply exceeded his ability.
That strikes me as quite plausible.
The particular plan that took him down, the attempt to acquire Dominion technology and shuttle it through the wormhole to the Typhon Pact, did seem to be reasonably well-constructed--the planners thought the Breen merchanter had the cargo capacity to transport the material, they expected the Dominion couldn't intercept the ship's transmissions or detect it, they thought that the warbird could sneak out of the Gamma Quadrant past DS9. It's just that things went wrong that the planners couldn't control, whether we're talking of the Pact underestimating Dominion capacities or of the timing problems with DS9's bombs. It's easy to imagine circumstances where the plan did go off successfully.
Tomalak, for his part, would have benefitted enormously from Sela's successful gambit. Kamemor and her regime probably would have been tossed out, but Tomalak? Maybe he could have become Emperor, after all. He probably was quite useful to Sela as a sufficiently conventional Romulan--a half-blood could almost certainly never have become Praetor, but a Romulan military officer of note with a close ally?