Yeah, each group count as one, i don't know why your game told you can't find the brush.
Did you make every new brush entity correctly?
This is Advent's anti-fighter's entity, there are three bursh entities need modify if you want to change. They are:
"HUDICON_FRIGATE_PSIANTIFIGHTER" and "HUDICONSMALL_FRIGATE_PSIANTIFIGHTER" can be found in the file HUDIcon-Ship.brushes
"INFOCARDICON_FRIGATE_PSIANTIFIGHTER" can be found in th file InfoCardIcon-Ship.brushes.