If you are going to try them together the Gem needs to
stay in the acc box for a few days. You need to be sure there is no aggression. An acc box would prevent injury or worse.
If you mean using a small PVC instead of an acc box, no it won't substitute. If things go south the yellow could kill the Gem
before it escaped to the PVC. Also you never want to release a fish where is has to stay hidden & stressed. One fish simply would need to be rehomed.
If you mean the scenario where the fish get along well (no aggression)
after using an acc box, then a small PVC would be fine.
TBH putting them together (despite the fact the Gem is a baby) would be too risky for me. Especially because of the size of the tank, 2 Zebrasomas** & the Gem is too tiny to defend itself. **Same species tangs are usually tried in odd
#s to try to curb aggression (so they don't focus on 1 on 1 sparring).
Whenever members post warnings & cautions, just keep in mind it is because they have been through a bad situation themselves. I had a Whitetail Bristletooth in a 300 gal that turned murderous after it matured & it was not with any other Ctenochaetus tangs. Tangs are territorial by nature. So if you are going to try them together use the acc box & let us know how it worked. I had a Gem Tang & it had a great temperament.