The double action trigger pull of a .22 rim fire will be noticeably heavier than your model 66. It takes a heavier spring to consistently ignite the rim fire cartridge. Regardless, S&W .22 rim fire revolvers, old and new, are a joy to shoot.

The difference in trigger pull between a 10 shot and a 6 shot .22 rim fire will be negligible or nill.

re: " Would shooting it be good training for shooting my bigger revolvers, or should I get an old timey six shot?"
In my opinion the more challenging trigger pull of the rim fire revolvers, either 6 or 10 shot, helps train you to maintain better trigger control and makes it seem easier to maintain good trigger control with the center fire S&W revolvers.

Definition of trigger control at militarytraining.net:
Trigger control is the ability to move the trigger to the rear to allow the hammer to fall without disturbing sight alignment or sight picture. Definition: Trigger control is the manipulation of the trigger, allowing the shot to break without disturbing sight alignment.

__________________
Scoundrel & Ne'er-Do-Well