Observations form Brian Munie (USA):
As far as the pike blenny goes, I have seen him eat smaller fish (a guppie once) just to see if he would eat it. Primarily eats brine shrimp mysis.
Does not show any outward aggression to my other fish: Barnacle Blenny, Percula Clownfish, Cardinal fish, Possum wrasse, Leopard Wrasse, Brotolid Goby, Jawfish. However he is territorial with the other pike blenny. Both have their own tubes the male (I believe) which has more distinct orange colors on the throat, will take any tube he wants. The female now lives in a tube on the other side of the tank. No real physical confrontation more acts of display by opening their large mouths and threating each other.
Also seen them do this behavior for protection when scared.
Occasionally with the barnacle blenny I have seen this behavior in a failed attempt to take over the tube the barnacle blenny was living in. The barnacle blenny had a much larger mouth that I thought, and actually even though he is about have the size of the pike blenny held his own, and defended his home.
Other references which I have reviewed and relay information in line with what I have observed:
Hemipteronotus splendens (Castelnau, 1855)
Hemipteronotus ventralis (Bean, 1891)
Xyrichthys splendens Castelnau, 1855
Xyrichtys ventralis Bean, 1891
Classification: Biota > Animalia (Kingdom) > Chordata (Phylum) > Vertebrata (Subphylum) > Gnathostomata (Superclass) > Pisces (Superclass) > Actinopteri (Class) > Perciformes (Order) > Labroidei (Suborder) > Labridae (Family) > Xyrichtys (Genus) > Xyrichtys splendens (Species)