From a bio load perspective you should be ok in my opinion as long as you have a good nutrient export filtration plan (refugium, water changes, large skimmer, etc). If you aren’t wanting a pretty intense water change schedule (10-20% weekly), or to run auto water changes, you may want to drop a tang or two off the list. They are poo machines.
I would also reconsider a few things if possible. Just a few things I would recommend.
The Regal Blue Tang (Hippo) will get a bit too large for your system long term, so I would scratch that off the list unless you have a plan for rehome it later or upgrade your tank in a couple years.
I would also steer you away from two Acanthurus tangs in a tank that size. The Powder Blue and Brown will have a tough time getting along and will end up stressing each other out a lot or injuring each other. Maybe you could swap the Powder Brown for a Tomini tang instead? That way you are staying away from two tangs in a system that size from the same genus with the same look and body shape. Plus the Tomini will stay smaller and contribute less to bio load.
You could try the six line with the Christmas wrasse, but it is a pretty high risk that the six line will be a jerk later or right when the Christmas Wrasse is added... it may also be a jerk to the blenny too, but it really depends on your specific six line’s personality. Sometimes they are okay, other times they beat new additions to death. I would probably recommend taking the six line out honestly, but it’s your risk if you keep it.
Stocking order with the modifications I mentioned to avoid aggression:
1. Blenny
2. Christmas Wrasse
3. Copperband
4. Tomini Tang (or skip it)
5. Yellow Tang
6. Powder Blue
7. Mandarin Goby (needs a mature tank with lots of microfauna).
