Because you are using the term Label Legend", which is a term used in Vectorworks, I'm assuming that's the drafting program you are using ?.
If so, 2 steps.
1) In the Label Legend manager, in the Spotlight tab, when you create a legend, there's an option in the middle of the first picture - Lighting InstrumentLayout Symbol, used to select the type of symbol you want to use for that legend.
2) In the Spotlight tool - Find and Modify, select the instrument type, then choose a Legend to assign. As shown in the 2nd picture. The 2nd method is to select the fixtures you want particular legends assigned to, then go to Spotlight tab, Label Legends and Assign Legend to Instruments. The Find and Modify is global for an instrument type.