My my, you are right!I remember fighting him without summoning him before the Catacombs boss, I didn't even know you could summon him for help.
Sorry for the bad info! ^^Leeroy's invasion will happen regardless if he's summoned in the catacombs or not.
Note: You have to have placed the Lord Vessel at the altar and making the Yellowish/Red Fog gates disappear. Otherwise Leeroy will not invade you.
You also have to do it BEFORE killing Nito.