you could get away with a pair of blue throats in a 55 ive never seen one more than 8 inches in captivity. make sure you get male and female though...trust me, im talking from expirience....
If your still decided to go with a 55 galon tank you can easily put an Undulated Trigger in that tank for life. There are beautiful triggers that get to around 9-10 inchs...some sources say they get to around 12 inchs however I believe this number is inflated. As far a tank mates go you would want anymore then one other fish. Due to the fact that this fish should be aggressive, might a suggest a stripped spiny box puffer, or a webed spiny box puffer. Both of those puffers max out at roughly 10 inchs so you'll be in good shape...Obviously if you can get a hold of a bigger tank these two fish will do wonders and provide you with a great amount of extertainment.
Nigers swim huge distances in the wild so certainly arent suited to a 55. Unfortunatley you will have to face facts that no trigger is suited to a 55 for life.
i disgree...picasso triggers stay under 10 inchs aswell as bursa triggers, in a species only tank you can also fit a undulated trigger in there I know some one who has had a 55 galon with a undulated trigger in it for 13 years so trust me u will be fine.