The main thing that bugged me about "Yesterday's Enterprise" was that the Galaxy class was in use at all.
We have no idea what the full internal structure of the alternate Galaxy
was like. Tasha said it was capable of transporting over 6,000 troops. For all we know, the whole ship was completely different on the inside than the version we know. The quarters could all be spartan barracks, with the rest of it turned into weapons testing labs, training grounds, phaser control rooms, etc.