Well the general rule of thumb that i think you will see printed in alot of books now a days is 1" of ADULT fish per 5 gal
of tank capacity.
Now this is a general rule and does not fit every circimstance. Some fish have more of a bio load than others because they are 'messy' eaters. Other fish need swimming room and just because they fit the general rule does not mean tthey are a good fit for the tank size.
With that in mind a 75 gal
tank would give you 15" of fish to work with. Now both the tangs can get up to 5-6" or larger in size as an adult so that would be 10-12" of fish. The damsel would get upward to 3" maybe 4" so thats 13-16" of fish at adult size before we figure in the trigger.
Tangs also like and need good swimming room so I would say two tangs in a 75 is probably the max for tangs in a tank that size.
Other factors to consiter when stocking the tank is the amount of biological filtration you have. Aswell as the general compatability of the existing and desired fish.
If your looking for maybe some more recently published works please check our saltwater library.