1. it is going to depend on several things (listing in descending order of importance):
a. the type of tangs, not all tangs are created equal, especially in regards to aggression.
b. the individual personalities of the fish. sometimes fish thought of as "conventionally peaceful" can be terrors, and vice versa.
c. the actual dimensions of the tank
d. the layout of the tank including the shape/configuration of rocks, lines of sight, etc...
same answer goes for the blennies as well. some will cohabitate better together than others. it would be best to select your short list of fish, then ask the question "how do these specific fish tend to get along..."
2. No.
i would not recommend any kind of dragonet to a beginner, nor any type of obligate feeder, or specific feeder. i actually keep a mandarin dragonet (Synchiropus splendidus), scooter blenny (Synchiropus ocellatus, it is NOT a blenny as the common name would imply), and a Rainford's Gobi (Koumansetta rainfordi, aka Court Jester Gobi, and actually in the Gobiidae family), and i wouldn't advocate anyone keeping them who was new to saltwater.
here's why:
the first two on that list, S. splendidus and S. occellatus, are obligate feeders. they require a large population of benthic critter, primarily copepods, to survive. forget what you've heard about teaching them to eat frozen, or teaching them to ride motorcycles through firey rings, it's not a good plan, and more often than not they will not have their nutritional needs met by discreet feedings from the keeper, if you can get them on frozen at all. they function more like a hummingbird in many respects, constantly grazing and not really retaining the food in any real way. so a population of benthic zooplankton is a requirement for their care.
same story for the K. rainfordi. they like to sift sand. mine is constantly gobbling little mouthfuls of sand, and spitting them back out through his mouth and gills as he sifts out the little zooplankton that live there. similar to the dragonets, he grazes constantly, and even though i have had him for a little over two years at this point, i still haven't gotten him to reliably eat frozen food, or other prepared foods.
given the special dietary requirements, and generally fussy nature of these three, they're best left to someone who has a bit of experience. my general rule of thumb is one year in the hobby for any of these guys, for two reasons:
1. it gives your tank time to mature and stabilize the populations of zooplankton required to make up the primary food source.
2. it gives you, the keeper, time to make your worst dumb mistakes, learn from them, and hopefully not make them again.
but wait! i can hear you shouting at your screen: "but I won't make those n00b mistakes! i've read so much, and i'm a super genius!"
trust me, you will. we all did. i made sooooo many stupid mistakes my first year in the hobby that i lost count of all the dumb stuff i did! things that would have easily been the end of more delicate animals like dragonets or small gobies.
but all hope is not lost!
in a 150 gallon tank you have the advantage of size, and over time you should build good populations of the zooplankton these critters need to survive.
so no worries, you will get there. but start off slow. start with species of fish that easier to care for. get your husbandry and maintenance routines under control, get your really dumb mistakes out of the way. learn your tank, and what works and doesn't work for it.
then after you've been running and stable for about a year, start thinking about the more challenging fish you can start to keep. then you can go in to it armed with more experience and knowledge, as well as a big population of pods, and have the confidence to know that you're stacking the deck in your favor.
good luck.
