I think that playing chords and understanding harmony from the framework of a piano is the real purpose of learning how to play the instrument for others, not hammering out Mary Had A Little Lamb with one finger.

To actually play chord progressions requires some technique and understanding of how chords are built, and you would be better off figuring out how to play your instrument to some level of comfort than jumping into something entirely new alongside it. Not to mention that for most people it isn't a mad rush to learn everything as fast as possible and motivation/fun is the most important factor.

I say this as someone who was competent on guitar and bass and then learned keys. I would do it the way I did every time, albeit maybe faster.

>If you're playing a brass instrument, a piano is really helpful at the start when you're building technique so you can hear what note you should be playing rather than hoping you're hitting the right partial.

Sure, I can agree with that, but I don't know if that's really playing the instrument so much as having one around as a tool.

They are completely different in terms of technique and that is the first stumbling block for new musicians.

I agree, but take things one step at a time. Let it happen when it needs to happen. It can be easy to get overwhelmed by unintuitive and foreign concepts when you start learning music and it's OK to just focus on the basics of, say, guitar first, and then once you feel like you have some foundation you can jump into keys. Do it when it feels natural.