You have stated that the product is 'pre-cooked' if this is the case, then the patties are cooked, and therefore only require reheating in the fryer before consuming? Be careful by your wording as if you are audited and use the words pre-cooked it could imply you have cooked the products to a point where they are microbiologically safe to eat.
For coliforms and histamine, I would use the UK HPA 2009 guidelines on: http://www.hpa.org.u...e/1259151920651
I know these are UK guidelines, however they can give you some good support and background information. You will also need to conduct analysis for histamines and should conduct these based on a risk assessment. Look at your country and your supplier, how frequently histamine is detected in tuna, what are the counts etc.... this can give you an idea of the frequency of testing. You obviously don't want to overkill this as it can get very expensive.
I oversaw a BRC audit involving Samosa products. These were sold as uncooked, however had been fried during manufacturing. On some occasions the probe temperature reached 75°C however on other times only 55°C. The frying was for colouration only, not food safety because it was sold with "cooking instructions" as you have stated. We received a MAJOR non-conformance as the auditor stated that because pasteurisation temperature could be, and was achieved on some occasions, this should be consistent.