Well you really should be doing 3x5. That in itself would cut down the volume and could help some. The higher reps mean handling less poundage and although you could still add strength that way (especially for a beginner), you would most probably not be able to progress in strength anywhere near as long.
Now don't quote me on this but I'd say that it's volume that is going to have the longest running fatigue and will be the most likely to affect your legs in hockey (at least until you are handling some really big intensity at the volume your at now...meaning 3x5).
To put it another way, it's more likely the amount you are doing that is affecting your legs in hockey rather than the weight you are using. So drop the volume. Even though intensity certainly plays into it, meaning you could do "endurance reps" of say 20 or more with no problem, I've a feeling that a couple three sets of 8-12 would be worse, not better, when you couple the greater volume with the frequency of the program. Doing strength work in itself is not going to bring down your endurance although I'm sure a lot of people think that it would. On the other hand adding in cardio with the idea idea that your legs need more endurance could just mean more work, more fatigue, less time to recover...you get the picture. That would stand to reason if the "cardio" was of a high intensity nature and therefore close in nature to what you are doing now. Maybe slow steady state cardio wouldn't be a problem but the question is would it really help. I'm pretty sure Hockey isn't like a slow jog
I really don't know anything for sure. The one thing I can be pretty definite on is to drop the volume on the Rippetoe.I mean your squatting 3 times a week. But no I don't think you need to choose one or the other at all. I'm positive that Rip himself has put similar athletes on this very same protocol many a time