According to John Ayto & John Simpson, The Oxford Dictionary of Modern Slang (1992), both journo and muso originated in Australia. Here the dictionary's entries for the two terms:
journo noun orig Austral A journalist, esp. a newspaper journalist. 1967—. TIMES Journos who work with the written word are seldom at ease with spoken English (1985). [Shortened from journalist + -o.]
muso noun orig Austral A musician, esp. a professional one. 1967–. K. GILBERT I used to be a muso and a hustler from the city but I'm a tribal man too (1977). [From musician + -o.]
Eric Partridge, A Dictionary of Slang and Unconventional English, eighth edition (1984) doesn't have an entry for muso, but it dates journo to circa 1940:
journo. A journalist: Aus.: since ca. 1940. (B.P) The almost ubiquitous Aus. -o, taken over—in part, at least—from Cockney immigrants.
Partridge's remark about the "almost ubiquitous Aus. -o" suffix corroborates curiousdannii's comment beneath the OP's question that "In AusE there are loads of diminutives like this."
A couple of booklets from the National Museum of Australia—Aussie English for Beginners (2002) and Aussie English or beginners Book Two (2003)—identify several other -o slang terms used in Australia: arvo (afternoon), bizzo (business), compo (worker's compensation payment), garbo (garbage collector), rego (motor vehicle registration), and smoko (smoking break). Both bizzo and garbo seem to meet the OP's specific request for "other shortened (collective?) [occupational] nouns like these."