If we remove Star Wars: The Clone Wars - The Lost Missions (Unfinished Season 6) from our view, what was original thing Lucas wanted to tell about this? Why did Palpatine order a Clone Army for his grand plan? Couldn't any other army do? Definitely, any army would follow Order 66. Also, there was a possibility that Jedi Council and Senate wouldn't accept that Clone Army. After coming into power, couldn't he order for a grand army creation or simply officially order a clone army?
Let's look at the Attack of the Clones script for answers
LAMA SU: We modified their genetic structure to make them less independent than the original host. As a result they are totally obedient, taking any order without question.
OBI-WAN: You mentioned growth acceleration...
LAMA SU: Oh yes, it's essential. Otherwise, a mature clone would take a lifetime to grow. Now, we can do it in half the time. Those items you saw on the parade ground were started ten years ago, when Sido-dyas first placed the order, and they're already mature...
LAMA SU They're immensely superior to droids, capable of independent thought and action.
Why did Palpatine order a Clone Army for his grand plan?
Palpatine wanted an army that was intelligent, effective, and obedient. The clone army fits all of these criteria - and could be created in secret and revealed exactly when he was ready to use them.
Couldn't any other army do? Definitely, any army would follow Order 66.
A droid army is the only other type of army that would be 100% obedient. The clone troopers spend years fighting alongside the Jedi. The average soldier is not going to turn and kill their friends without hesitation just because their commanding officers/commander in chief tells them to. The clones were genetically modified to obey any order, and no order could supercede the Chancellor's.
Also, there was a possibility that Jedi Council and Senate wouldn't accept that Clone Army.
This is a possibility - but Palpatine engineered events so that the Jedi would discover the Separatist Droid Army and the Clone Army at the same time. With no other standing army to battle the Separatists, a clone army genetically bred to be loyal to the Republic is going to be awfully hard for them to turn down.
After coming into power, couldn't he order for a grand army creation or simply officially order a clone army?
He could order the creation of a non-clone army, but would have to deal with issues of loyalty and quality. He could order a new clone army, but as Lama Su explains, it takes time for them to be grown. The clone army was commissioned 10 years ago, and is only now ready. That's far too long if they are to fight the Separatists.