Fighters are naturally agressives towards other fish - you're lucky it hasn't attacked any of your guppies before this.
In saying that though, I have my boy in with 2x silver dollars, 2x angels, 4 cories and khulis and yep - he may be a wee boy but he's ruling the roost
My concern is moreso that the angels will attack the fighter's fins, but so far they've left him alone - will keep an eye on things though.