1) Gaara aka "one tail" and Naruto aka "nine tails"....
It's pretty much unanymous that kyubi would be stronger than shukaku, but yes Gaara is probably much stronger than Naruto. I don't think this necessarily has anything to do with training (because well, who could train Gaara to control his sand?). This has to do with the type of seals they have for their demons. Gaara's seal grants his demon greater access to him, thus allowing for the passive defense and instinctive sand control. Naruto's demon has like, practically no access to Naruto except in times of vulnerability, whether it be extreme emotion or serious injury.

2) Strength vs Rasengan....
Under normal circumstances, you should be able to defeat an enemy just as easily with a kunai as with any of the advanced assassination jutsus, however there are circumstances where sheer power is needed. When Tsunade fought Kabuto and Oro, would her punch have been able to inflict as much damage to Kabuto as Naruto's rasengan did? The rasengan does more than just break some bones, it's like a hollow-point shell, where upon impact it expands and inflicts massive area damage throughout the enemy's body.

3) Oro's strength among Akatsuki....
I think Oro could take Kisame.