Mixing hardwood and softwood - bearers, joists, studs, noggins

I want to build a granny flat and am looking at materials for the bearers, joists, studs, noggins etc.

The hardwood bearersand joists will be either spotted gum or stringy bark. I gther that either is acceptable and complies with the Australian building codes.

This then got me wondering if I should (must?) continue upwards using the same hardwood e.g. studs and noggins or can treated pine be used for the studs and noggins. I guess I'm not sure about whether one is allowed to mix hardwood and softwood in this way? Perhaps expansion and contraction rates/amounts would be different.

For the surfaces, I'm thinking of using:
- Scyon Secura (JamesHardie) for internal flooring
- 180mm Scyon Linea(James Hardie) for external wall cladding

and, I guess,
- HardieBrace for bracing


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