Wanting to play a Space Marine in a standard D&D setting is not equivalent to a Druid wanting to don a breastplate for a disguise, and neither is equivalent to the players of a half orc and an elf wanting their characters to get along.
If something doesn't exist in a setting, then it doesn't exist. It's a restriction of the meta. A DM can make allowances for a player (okay, you can be the only elf in the world but you also have complete amnesia) but is under no obligation to do so. If we agree to play 5e, that means we're not playing Warhammer 40K. A player who insisted on playing a Space Marine after agreeing to play 5e would be in the wrong, IMO. A player who politely asked the DM whether there's any way he could play a Space Marine because he's got a really cool idea for a Space Marine but accepts the DM's decision like an adult regardless of the outcome is fine.
On the other hand, the druid limitation against metal armor is an in-game restriction deriving from the game's lore. I suppose the DM could come up with their own explanation for why druids CAN'T wear metal armor, such as a Geas woven into the fabric of their pact with the land, that literally makes them incapable of forcing themself into metal armor. It's a touch heavy handed for me, but as long as the players have some understanding of the in-game justification, I'd say it's fine. Personally, however, if I were to go this route I'd simply say that they can do so but lose their druid powers for X amount of time. I'd much rather give them the option than not, even if the option is so terrible that 99 times out of a hundred they'll choose not to do it. (As I've established however, my preference is to simply have in-game consequences for breaking the taboo.)
Lastly, you have half orcs and elves hating each other, which is really just an in-game stereotype. (Unless, of course, you have some physiological or magical explanation for why they always hate each other without exception. Something like that might be interesting in one particular instance, but trying to justify all behavior via this route would almost certainly come across as extremely heavy handed.) Assuming that it is a stereotype, there will be individuals who do not conform to the stereotype. I see no reason why the PCs cannot be such nonconformists. I would expect adventurers not to conform to the norm more often than not.
Humans and halflings only is a meta restriction. Druids won't wear metal armor is an in-game lore restriction. Both are fine but they are not the same type of restriction. If you want your druids to never touch metal armor, in my opinion you need a better reason than "because I said so" or even "because the book says so". It could be "because you will instantly die" or "because you will permanently lose all of your druid powers" but there needs to be something. Otherwise, IMO, the DM is overstepping their role by dictating the behavior of PCs. The DM creates the world and controls all its workings, but the players should always have final say in what their characters do (barring magical compulsion and the like). The DM can certainly advise (this is why druids don't wear metal armor and that is what you've been told will happen to a druid who does) but should never take away control of a player's character without a strong in-game justification (magical compulsion).
