Hijacking stats is great, it's hugely encouraged for 4e too since the mechanics don't give a shit about the flavour; all that matters from a mechanical standpoint is that the numbers are right.
Also, the DMG has a fun little section on creating monsters from scratch; the math is pretty simple and with it you can throw together a pretty decent monster in about 10 minutes, or less with practice. I've gotten so good with the underlying 4e mechanics that i can design monsters on the fly (something which still slows me down sometimes in 3.5).
And good doesn't always co-operate. You might be trying to steal the widget of cleansing from a gold dragon's horde and need stats to try to bluff your way out. Or that a paladin order will destroy your ability to get a peace treaty and need to be stopped. Simply stating that good creatures don't fight good PCs has a significant impact just by itself.
That's not technically what they're saying; what they're saying is that on average, 'Good' monsters will not appear opposing the PC's all that often. You can come up with a situation to justify
anything, but WotC are looking at the averages, and on average most of the Good monsters in the previous Monster Manuals were not used very often, the Devs felt that it would make more sense to remove the Good monsters from the first Monster Manuals and address them at a later date in a specific 'Good Aligned' Monster Manual, and frankly i agree with them. Most of the time i have absolutely no use for good creature stats, i'd much rather not pay for a Monster Manual that has a bunch of creatures i can't really make all that much use of.
Also, Metallic's arent Good anymore; they're Unaligned. Although like everything else they can be Good or Evil, just odds are that any Metallics the PC's face will be Unaligned, rather than Good.
Also why are you stealing from a Gold Dragon? (The situation you just suggested would also be solved with a Skill Challenge, not a combat Encounter.)