I understand what you are saying and your right, rockling are very confusing and this time last year I was very confused, but the species hunt put an end to that. Once I'd caught a few and seen others that where posted then the diferences became clear to me. The best way to ID fish in my opinion is by looking at photos of the fish when not in the water, because thats when we see them, a lot of fish look totally different when there in the water.
Looking at the pictures I posted, I know the 5 bearded isn't that great but you can defiantly tell the difference and when you catch one they are slimy like a slug. The shore rockling isn't but it does look very similar to the 3 beard but then the colour is the give away.