1. Should Australian plants be planted out when small?

Quite apart from the economic factor that smaller plants cost less, it's commonly advised to use small plants wherever possible. However, this is mainly because smaller plants are generally less likely to become "root bound" by being kept in a pot for an extended period. For this reason larger plants may be more likely to be slower to become established when planted out.

If you can be sure that the root system of the plant has not developed into a tight mass in the pot or if you are prepared to carry out some root pruning before planting, there is no reason why larger plants can't be used.