the theroy is not true and this is my prove:
itachi joined akatsuki b4 he killed his clan because i at the end of episode 135, one of the akatsuki states how itz been 7 seven years since they met, at that time, itachi would be 11 or 10 years old?
and in that 'theory' it says that he heard about the MS from his father and THEN joined akatsuki

if itachi needed a third MS user(sasuke), then y would he invite sasuke for a final encounter (and sasuke doesnt even have the MS)?

and itz strange that tobi/uchiha madara joined akatsuki so late, i mean, akatsuki isnt exacaly the newest organization

and the theory is also saying that shodaime and madara fought because madara sought power? thats kinda unreasonable...all shinobi sought power

and is the kyuubi the type to sit down with someone to have a talk and kindly lend power to someone weaker than them because they asked? i dont think so