Aircraft Weight Question

Consider the following aircraft:

Seats ---------------------------------------------- 2

Empty Weight (typical) -------------------- 920 lbs.

Maximum Gross Weight -----------------1320 lbs.

Useful Load ----------------------------------- 400 lbs.

Payload with Full Fuel (18 Gals) -------- 292 lbs.

Suppose I wanted to add additional equipment to this aircraft. Obviously,
this would raise the Empty Weight, and lower the Useful Load and the
Payload.

Is there any sort of "minimum" Useful Load or Payload that must be
maintained when equipment is added, or is it simply a matter of operating it
within the "weight envelope"?