According to this page
http://www.red-duster.co.uk/royden2.htm "Zinbu Maru" was originally the British "Indrani (2)" of 4994 GRT built in 1894 and eventually sold to the Japanese. She became a war loss in 1942, though no further details re where/when/how. So far I haven't found a photo of her so I can't comment on whether she was modelled accurately or not.
"Paula"/"Arare Maru" was used as a shuttle tanker between Palembang and Singapore after the Japanese salvaged her, though she may have made an occasional voyage to Batavia or other destinations in the area. The devs modelled her quite accurately, though they did forget her mainmast!!!!
While they WERE one-offs in the Japanese Merchant Navy, as they are already in the sim why not use them
? Ditto for the "War Melody"- as "Ryuun Maru" she was most probably a one-off as well.
Adding the war-built standard types will mean modelling them from scratch- unfortunately my skills are nil in this regard, otherwise I would have started on Hr.Ms. "Java/Sumatra" long ago.