Sidious uses Form VI, also called Niman. This combines Forms 1, 3, 4 and 5, using each one in overall moderation.
This Form was created to give the Jedi a style that would be sufficient for most combat situations, but would give them more room to train in negotiations. It is therefore ideal for Sidious who is a politician.
Form VI is often considered to be less effective than other Forms. The fact that Sidious uses it to such devastating effect shows his power and skill.
I see your point, but as I say, it's the ideal Form for Sidious, considering how much time he has to spend in negotiations. He is in charge of both Republic and Separatists, so he is clearly very busy.
Besides, it just shows how lethal he is that he is able to use 'the weak Form' to cut down Master Jedi like wheat.
For form VI:
All Jedi using that form on Geonosis DIED in the conflict. Actualy that form is nothing I would use in a real battle.
For Sidious: It's only an assumption that he used that form. It won't fit his character and actually his moves in ROTS doesn't seem very much like form VI (watch the Jedi on Geonosis moving and compare that to Sidious).
I'd say he would more likely use form V (aggressive movements, wide swings) or VII (Darth Maul used that one). As Sidious did show Maul how to fight he would probably be a form VII (Juyo) user. That also fits his characters and movements more than form VI.
Still that is also an assumption.