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.
Depends on circumstances. Zannah would train Cognus. And season 4 of TCW Asajj mellowed out a bit. Belia Darzu did work with other Sith, but they were all afraid of her and had her assassinated. Revan is a natural born leader. he could keep them all in line and functioning together. According to TOR he supposedly trained "A thousand dark apprentices". That's pretty nifty. Sith can get along if there's a mutual goal in mind they all need to see through to completion.
I picked them because this is supposed to be a Strike team, not "Most Powerful side-liners". Sidious manipulates from the background. he doesn't directly engage in conflict unless he's assured he'll win. Vader is slow and doesn't work with anyone other than Stormtroopers. As Anakin he may have had some sneaky moments, but as Vader he has no tactical ability for use in a Strike Team. Caedus to me was more of a joke than anything. he got to tick off Luke then get spanked for it.
I mostly chose people for different talents. Revan isn't just a power monger, he's a leader and tactician. Belia has things she can use to really cause havoc and leave opposing forces in disarray. Ventress can sabotage areas as needed or fight off enemy forces. Zannah is more of a defender, but her power as a Sorceress is handy as she mainly focuses on twisting peoples minds. Then there's Cognus who can find and discern the best route to take to their goal with the highest rate of success. These five working together would make an excellent team.