Security for Units 731, 100 and their associated sub units was provided by regular units of the Kwantung Army (IJA Manchuria), local Manchukuon auxiliaries and the Imperial Japanese secret service the Kempeitai. The armament of these personnel would likely include most any of the small arms in service with the Imperial Japanese Army during that period. The Type 99 and Type 38 rifles would be prevalent types while offices and security officials would likely be found with Type 14, Type 26 or Type 94 pistols. Obsolete small arms such the Type 30 rifle, as well as captured arms such as the Chinese type 24 could also be expected to be in the hands of auxiliary troops. The use of submachine guns was restricted by lack of industrial capacity and survivor accounts frequently recall the use of rifles and their bayonets in various experiments as well as in the execution of prisoners so while present they would be unlikely to be used in large numbers. Barring access to a specific ToE, personnel assigned to the Units would likely have been lightly armed with small arms as befitted their assignment guarding, moving and disposing of detainees

Personnel from these units and the Kwantung Army were intimately involved in the rounding up of human test subjects and the field testing of biological and chemical weapons on local populations. They collaborated in the development of artillery and aerial ordinance for the deployment of these weapons and then supervised their testing against the civilian population of China in the field with these experiments being facilitated by the Kwantung Army. Here you would likely find the use of machine guns like the Type 11, Type 96 and Type 92 (possibly used for facility defense). For the purposes of your book (especially if Fiction?) armored cars, some aircraft, light tankettes and possibly even an armored train bear consideration, depending on the scope of travel and activities of the actors involved.

Holmes, Chris Spores, Plagues and History: The Story of Anthrax, Dallas: Durban House Publishing, 2003 https://books.google.com/books?id=irrr1OfxhBAC&pg=PA145&lpg=PA145&dq=unit+731+security+force&source=bl&ots=yq-4kRwf9d&sig=2T_Cj602F4Hr-rVghj6yx9tMzRU&hl=en&sa=X&ved=0ahUKEwiFyKaw6IzOAhVS92MKHaN_DPsQ6AEIMDAD#v=onepage&q=unit%20731%20security%20force&f=false

Jing-Bao, Nanyan Guo, Mark Selden and Arthur Kleinman, Japan’s Wartime Medical Atrocities: Comparative inquiries in science, history and ethics, New York: Routledge, 2010 https://books.google.com/books?id=mYDFBQAAQBAJ&pg=PT83&lpg=PT83&dq=unit+731+kempeitai&source=bl&ots=eQAssTev6S&sig=jkmzljpaMM1LCmTV_FgjbhyaxJQ&hl=en&sa=X&ved=0ahUKEwjm3fHr6YzOAhVY6WMKHe4yA6sQ6AEITzAH#v=onepage&q=unit%20731%20kempeitai&f=false