Yes you can take power from two of the three phases in the 277/480V panel. Yes you can use a single phase, 2-pole breaker in that panel to feed the transformer. Yes you can use a 225 amp breaker, but I would not. On the primary side, a 50KVA transformer would have a full load current of 104 amps. Per Table 450.3(B), you can protect the primary at 250% of that, and 225 amps falls within that range. But I tend to prefer the protection to be closer to 125% of FLA. So I would select a 150 amp breaker.
Per 450.14, the disconnecting means for the transformer (i.e., the breaker to which you refer) must be either in sight of the transformer or lockable in the open position. The phrase "in sight" means visible from and within 50 feet of. There are a few other important statements in that article, such as a reference to the rules for locking the breaker. I don't have the ability to paste a quotation for you. Perhaps someone else can do that.
