Jango? No. Boba? No. Cad? No.
Sidious is not a "strike team" type of character. Neither is Vader. Pre-Suit was even worse because he was just a hyped up emo. HK-47 is good at assassinations, but is not exactly the quiet type. Sion is just a killing machine who can't die as long as he's on a dark-side aligned world or a place of heavy dark side energy, and even then he can die if he no longer possesses the will to keep himself alive.
You want a Five person strike team who can do the job? Well, lemme think a moment.
Darth Revan: Intelligent, cunning, tactical brilliance and powerful.
Darth Zannah: Charming, cunning and a powerful Sorceress. Has managed to get into the Jedi Temple without being discovered.
Darth Cognus (aka The Huntress): Excellent Foresight. Natural Force Disrupting talent. Highly Skilled.
Asajj Ventress: Quick. Skilled. Proven effective at infiltration.
Belia Darzu: Intelligent. Sith Alchemist and Shi'ido, which means she can take virtually any form or appearance, making her superb at infiltrating and tricking the enemy.
Fair point, but you have left yourself exposed if there are Ysasmari in place. also, five sith would NEVER get along. Two could, with a third being an apprentice, but i think more than that would get a little out of hand.