I don't know that there is any real 'trick' to Googling, you just need to think out different and inventive combinations of keyword searches, peruse the results, click on the links and read through the information . . .
If you're faced with a page with a large amount of text to wade through, hold down Ctrl and F and type the keyword/s into the search (Find) box.
A couple of book references below which may be helpful. There does seem to be an array of conflicting information regarding Stewart & Co. I'm not aware of a published company history which would help to clear things up.
"Robert Stewart was a commercial photographer active in Australia. He had studios in: 267 Pitt Street, Sydney 1863-64; 396 George Street, Sydney 1867-68; 348 George Street, Sydney 1868-70. From 1871 Stewart also had a studio at 217 Bourke Street East, Melbourne. This studio seems to have become known as Stewart and Co. Stewart and Co. occupied various studios in Bourke Street, Melbourne, until circa 1900 (Davies and Stanbury 1985, p.235).
Sources: Davies, Alan and Stanbury, Peter (1985), 'The mechanical eye in Australia: photography 1841-1900'. Melbourne: Oxford University Press."http://www.lib.cam.ac.uk/rcs_photographers/entry.php?id=434
"Stewart & Co. operated photographic studios from various locations in Bourke Street, Melbourne, between 1871 and 1915. Ref.: Australians behind the camera, early Australian photographers / Sandy Barrie, 1992."http://catalogue.nla.gov.au/Record/2421932
The State Library of Victoria has an online collection of photographs and cartes de visit by Stewart & Co., and possibly your two other photographers. Might be worth looking through them.
I keyed in '284-286 Bourke Street' only in the 'Search For' field, and the five results for Stewart & Co are estimated dates c. 1896-1915. One is inscribed with an '07 date.http://search.slv.vic.gov.au/primo_library/libweb/action/sea...
Alos, Historic Australian newspapers (search) might be helpful in locating Studio advertisements at a particular address?http://newspapers.nla.gov.au/ndp/del/search?adv=y
You might be able to narrow down the probable date of an image by the 'fashions' of the time - the clothes the subjects are wearing, their hairstyles, poses, studio props/backdrops, photo mounts, etc. This, of course, would require some research.