I've found that engine bolts are all Metric, while Cylinder Head and Exhaust bolts are SAE. I have not tackled the harmonic balancer.. but if it stops snowing sometime today, I can go out and see.. I think I have up to 25mm.. and the SAE equivilant.
If GM says you cannot reuse the bolts, don't reuse them. If they don't say anything, then reuse them. The Head Bolts are "Torque To Yield" bolts, which are a one time use only bolt [both my FSM and Haynes manual say this]. Check your manual [FSM first, Haynes or Chilton'* second] to confirm. If you don't have one, go pick up a Haynes or Chilton'* manual, they are like $10, and they have good info.. well, for the most part [many will disagree with me].
'89 SSE - GX3 -190k, 155k on body - Back for another ride.