I would prefer to set the colours that I see, makes it easier to tell who is a commentor and who is OP

That's not quite as straightforward as you think. These are emojis, they ARE text, just like letters from various languages...etc are also text.

It would be a hell of a lot easier just to ban non-ascii characters than to start changing the font size based on each individual character. This would also be something that every single client would have to implement themselves which they definitely would not, the rendering performance implications would turn it into more work than you would think.