Hi Sam,
Other than the Ultralight, any model will handle a 50 lb dog without popping. For alpine lakes, I would probably choose an Atlin (the largest size), because it’s wider in the foot area so there’s more room to share between the two of you. If you were going to be paddling with the pup for hours at a time, I’d consider a Skeena XL for even more room, but on alpine lakes I doubt that would be necessary, and the extra bulk and weight might not be worthwhile.
Cheers,
Matt