A 5 cell pack would be ideal. If your charger can take it you could just unsolder a cell from a 6 cell pack.

My second choice would be the [link=http://www.horizonhobby.com/Products/Default.aspx?ProdID=CSECCBEC]CC BEC[/link]. This little gadget will take the voltage from your 6 cell pack (all the way up to 20 cells if you want) and keep it at a steady voltage. The voltage can be adjusted using the castle link (same one you need for many of their ESCs), so you could set the voltage at 5-6v or even 7v if you want to push your servos a little. Its a great deal for the price too.

If you have an ESC laying around then yes, you could plug the 6 cell pack into that, plug the ESC into the battery jack on your receiver, and plug your servos into the receiver. Seems a bit bulky and ghetto but it will work provided your BEC can provide enough current for your servos. What servos and ESC are you planning on using?