If the point is to learn about Szayel's abilities, the identity of the bodies aren't that important as long as they're hollows/arrancars who fought Renji and Ishida. But you're right that Renji and Ishida recognized the bodies which narrows down the possibilities. In any case, if the next arc will feature Isshin as a key character, the bodies had better be connected to Isshin in one way or another.
Another reason why I want to learn about Isshin's past is Urahara. I believe Urahara has a key role in the events that happened when Isshin lost his powers and/or married Masaki. Ginjou told Ichigo not to trust Urahara which means Urahara did something questionable to Ginjou. Urahara thinks it's not his place to come clean and tell Ichigo everything but he urged Isshin to do so, which is why I expect to see a confession from Isshin soon.
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