Explain Shishu, Madara, Obito then....
Shishui and obito never had Susano'o. Or, at least, it's never hinted they did. Seeing as they didn't have ama or tsuky, the fact they didn't have susano'o would strengthen the point you need ama and tsuky.
As for madara, nothing says he doesn't have ama or tsuky. Itachi's tsuky is a special brand which completely breaks logic, while sasuke's tsuk is simply an enhanced sharingan genjutsu(EMS genjutsu), same as with madara's own. In any case, madara's shown the ability to use a tsuky close to that of itachi after he siphons off gedo's power into his body.
We don't have any reason to believe he doesn't have Ama. He's a katon master.