If Kakashi and Maito Gai are Zetsus, I don't know why on earth are they fighting against Tobi. Tobi and Zetsus are supposed to be a team, not foes.
Moreover, Zetsu can only imitate someone down to chakra level on a single touch. Never has it been mentioned that Zetus can also inherit all the techniques of the people. The way Kakashi and Maito Gai are fighting, they seem to be quite adept with their Raiton and Taijutsu techniques, which is reassuring that they are not Zetsus.
Moreoever,
according to this page, there is only one ET of the 7 swordsmen left, which is probably nothing compared to the resources present in the hands of 3rd division. Hence, Kakashi and Maito Gai just took the liberty to deal with something more important rather than wasting time on trivial stuffs.
