I've got the 5A regulator. 1A isn't going to be enough for a car with two decent servos I think, as one of them can pull as much as 3A. They advise the 1A regulator only for electric touring cars that have one servo.
6.7V will be perfectly fine for most servos. A freshly charged NiMH pack can put out more than 7V. LiPo without a regulator would be overkill indeed, because a freshly charged LiPo pack can put out more than 8V.

I got myself a 240mAh pack for qualifiers, and a 500 and a 750 for finals (depending on runtime). Only tested it in the dry so far (still winter here) but it's amazing. The Servos are faster and stronger than with a NiMH pack that isn't freshly charged. And it's just amazing how light these cells are.

If you get the 5A regulator, just make sure you run PCM or Spektrum, as they discovered that it can interfere with FM and AM. They're developing a new 5A regulator now that doesn't have this problem I hear.