Thats because the vanilla heron is conflicting with mine, it will be fixed in my upcoming update where i add prefixes.
The boss fleet in my mod is rather tricky, rather than fleet size i'd say you should go with a very high level character that has a lot of skills on it in a heavily decked out capital ship or a very well rounded fleet that counters the royal armadas destroyer and cruiser spam, people have beaten it with a single ship or indeed a fleet being around level 35-40, the fleet itself is coded to ATTACK YOU once you hit level 30, it is of course VERY slow and easily kited if you don't feel that you can beat it by then.
It is definitely doable, but your gonna have to get a few skills in the tech trees to pull it of, which is the concept the fleet is designed by.
the recommended level to fight the royal navy from what i've heard around here is indeed, between level 35-40+, and once you do beat it, there is pretty much nothing in the game that can challenge you anyway so i give the player some rewards for beating it, a super cap or 2 and some special fighter wings to celebrate having beaten the boss.