First, research the fish you are interested in. The species, as well, not just the group. There are a few triggers that will be fine in a 90, but many--including many of the popular and commonly available ones--will grow larger than a 90 can support. For mixing--a lot will depend on the species you select. Some are not good with other fish, others are okay.
Tangs can be hard to keep. They need a big tank--a 90 is about minimum--and appropriate varieties of food. They'll be okay with the community approved triggers.
Boxfish--which one? Some are hardy and do well in aquariums, some are sensitive and tough to feed. Either way, I would not mix one with most triggers--box fish have toxins they produce in their skin, and being harrassed by a trigger could result in them nuking the tank (it's a defensive measure, so not a risk unless you have a fish that will harrass the box fish).
Please also take some time to review setup and cycling a tank. Starting this right will go a long way in being successful.