Simply, no. Tiring out/pushing Aizen to his "shinigami limits" doesn't automatically qualify him to be at the level of Squad Zero or even that he was stronger than the captains present during the war. Hell, even Ichigo injured Aizen, which was something Yamamoto didn't even really do, but everyone knows that he could. Kyōraku could probably keep up with him for a while if he wasn't under the effects of Kyōka Suigetsu. That's not to say that Isshin isn't captain level (high, no doubt; along with Urahara) but the battle between Aizen with him, Urahara and Yoruichi always seems to lead to this wild misconception that those three are stronger than the Gotei 13/captains who fought against Aizen, which isn't necessarily true.
Speaking of which, why is it that both Urahara and Isshin are or have been speculated to be "ex-Royal Guard members"? I never understood that one. Can they not just be former captains with exceptional talent? Seriously.