One can't simply summon second gado if one gado is already there in the form of Shinjuu.
O.T.-He can control them like that, but I doubt if Madara has that much amount of chakra to do so.
As far as we know Madara can only control 1 Bijuu at a time while Hashirama had Bijuus 2-8 when Madara attacked the leaf and never had to use them. Hashirama even took Kurama from Madara so no point of Madara doing that when Hashirama would take them back
If Madara will try to control all bijjus at once, the same thing will happen to Madara like it happened to Obito. Obito was not able to control all bijuus together properly with rinnegan.
Also if Madara will try to control tailed beasts, I am pretty sure that the Kages, Shinobi Alliance & Edo Hokages are not going to sit idle and just watching this.