first of all, you will want to make a more drawn out way to FX, fust pick them all, then run an integer A loop that takes one, adds the FX, then removes it from the unit group. Here, il write it out for you.
VARABLES NEEDED
FX(effects array, size = to at least 100 preferably)
i(integer)
Tempgroup(Unit group, no array needed)
ok, now for the trigger
TRIGGER 1
EVENTS
blah
CONDITIONS
blah
ACTIONS
pick every unit within (whatever) of (whereever) and add them to Tempgroup
For loop integer A 1 to the number of units in Tempgroup
pick random 1 unit in tempgroup
create FX (whatever)
set cast created special effects to FX(i)
set i to i +1
remove picked unit from Tempgroup
ok, that will store the FX for us, now we need to destroy them
TRIGGER 2 (SFX cleanup)
EVENTS
Every(whenever) seconds
ACTIONS
loops for integer A 1 to i
destroy FX(integer A)
(loops ends)
set i to 0
Hope this helped!
EDIT: ALSO! please note that even though other effects go away, they are still stored in memory and thus must be destroyed to prevent very massive memory leaks!! | 
