3Phase + Neutral, 208VAC distribution and conversion to 1Phase 120VAC
If you are having to ask then be clear that you are not qualified to carry out the work. If you are seeking understanding so that you can request a quotation for someone qualified to do the work then:


\$ V_{L-N} = \frac {V_{L-L}}{\sqrt 3} = \frac {208}{1.73} = 120 \ \mathrm V\$ so, yes, that will work. Generally the supply should be able to take an imbalance but there are too many unknowns in your question to answer definitely.
If the secondary is wired in delta than you have no neutral and the transformer is floating with respect to neutral. You cannot connect neutral to your load neutral.


Again, at 800 V AC this job is not for amateurs.