Hi David,
The issue as to how many servos you can use on an ESC has nothing to do with the motor, the BEC in the ESC uses a linear voltage regulator to drop the battery voltage to the 5V that the receiver/servos use. in doing so it creates heat, the geater the voltage that the regulator has to drop, the more heat is produced, so as the voltage goes up (more cells), the less current you can safely pull thru the BEC circuit. Here's a simple bench test to see if the BEC is the problem, first remove the prop, then arm the ESC with a fully charged battery, then proceed to wiggle the sticks constantly, working all the servos, if the BEC is overheating, this will cause it to show up, better on the ground than in the air you should be able to feel if it's getting hot.
If you took the time to look at the Parkbec link in my last post you will see that it too is rated at 1.5A, the difference here is that it uses a switching type regulator, with this type it can provide the full current rating up to the max recommended cell count.
Good luck,
Pete