There are several series of 757's. This is the reason you are getting some conflicting information if a 757 is a heavy or not. In the air traffic world, the 757 depending on the series falls into its own special category for wake turbulence separation or if it falls into the heavy category.
If the model in question is a 757-200 it falls into the special category which requires the following radar wake turbulence separation in a terminal environment:
Heavy behind a 752: 4 miles
752 behind a 752: 4 miles
Large behind a 752: 4
Small behind a 752: 5
If the model in question is a 757-300 it falls into the normal heavy category which requires the following radar wake turbulence separation in a terminal environment:
Heavy behind a Heavy: 4 miles
752 behind a Heavy: 4 miles
Large behind a Heavy: 5 miles
Small behind a Heavy: 5 miles, except on final where 6 miles is required. More info can be found in section 5-5-4f of the 7110.65
This only applies in a terminal environment, ie within class B/C/D airspace or if you are utilizing TRACON services. In an enroute environment, the rules completely change and you throw all those numbers out the window.
And to your frequencies question from the 7110.65;
2-4-1. RADIO COMMUNICATIONS
Use radio frequencies for the special purposes for which they are intended. A single frequency may be used for more than one function except as follows:
TERMINAL. When combining positions in the tower, do not use ground control frequency for airborne communications.
NOTE-
Due to the limited number of frequencies assigned to towers for the ground control function, it is very likely that airborne use of a ground control frequency could cause interference to other towers or interference to your aircraft from another tower. When combining these functions, it is recommended combining them on local control. The ATIS may be used to specify the desired frequency.