There is no such rule for a saltwater system. The maximum number of fish depends entirely on the fish selections and their behavior towards each other in the aquarium.
Far more important than the number of fish, is that they have adequate space to grow to a full adult size. The species you list can easily live between 10 and 15 years in captivity. Honestly, a 150 gallon aquarium is not all that large for a marine system. These fish will grow to a minimum of 18'', and some of these fish will grow even much larger. I would personally want to upgrade to a 280 to 400 gallon aquarium within the next few years.
In the mean time, i think you can comfortably keep 3 or 4 of these fish without aggression becoming an issue. I would suggest the Emperor Angel, Niger Triger, and Blue Hippo Tang. You will have plenty of space for them to grow until you are ready to upgrade your aquarium.
The Blue Line Grouper, Porcupine Puffer
, and Blue Jaw Trigger all grow at a very fast rate. Despite the willingness of hobbyists to keep these fish in common size aquariums, I feel they are best left for the largest of setups. To many hobbyists think they have achieved success if a fish lives for 3 years. I disagree completely, and for this reason i feel these would all make great fish to add to your future upgraded aquarium size, where they can live a long healthy life.
As to the Snowflake Eel, they are boring fish anyhow. Why even waste the space? You will almost never see the fish. But if you do, then yes, you have to factor in the effect on the bioload, although it will not take up swimming space, as you mention.