Metal dragons are already at max level, so no need to feed them. They're purely meant to be fed as is. The only one you might make an exception for are the gold metal dragons, because they can be sold for a lot of gold. I sold one for 30K.
Regarding some of the advice in the OP, I regret spending my first 15 magic stones on rare gacha pulls. The OP says that's really all the magic stones should be spent on, and only expand your monster box when absolutely necessary. Well, unlike with your initial tutorial rare pull, you can't reroll subsequent rare gachas. And even with special events percentage increases, chances are against you getting something truly great. It's possible, but if you whiff, you put yourself at a significant handicap in monster management. You quickly start amassing a bunch of monsters, and it forces you to fuse or sell monsters that you might not be ready to. It really limits your ability to efficiently raise your monsters when you can't stockpile a bunch of feeder pets and evolution materials. I've since been spending all my magic stones on box expansions, and I'm now up to 60. And it's made monster, and consequently money management significantly easier. I can wait until I have 5 feeder pets to fuse, instead of fusing only 1 or 2 because I'm out of room. The extra boxes have been far more useful than the 3 rare monsters I pulled, and it cost me less than half of the stones I wasted pulling rare gachas.
So, in short, I think it's much more useful to spend early stones on extra monster boxes. I don't know how many would be optimal, but I think 50 is a good number to start. And that's only using 6 stones! Only when you feel you've gotten a good number of extra boxes, that you don't feel forced into selling/fusing monsters that you'd rather not, would I suggest then spending stones on rare pulls.
Fully agree with the monster boxes thing.