You can do it in both forms by specifying the Precision, i.e. the total number of digits in the mantissa:
EngineeringForm[66666.66`7]
or
ScientificForm[66666.66`7]
If you want to present the mantissa with only one digit before the dot in EngineeringForm you must set the option ExponentStep equal to 1:
EngineeringForm[66666.66`7, ExponentStep -> 1]
