In regard to the ceiling thickness--I don't think I have enough clearance to pour 6" thick. If I did, I get 12.2 K-in. Then S=?? Do I use 0.66 and 50 in the M/Fy equation? (Is 50 the concrete weight/ft^2?)?

Answer: Fy is the yield stress of the steel I beam which is 50,000 psi or 50 ksi. Sometimes for older I beams, Fy is 36 ksi. Check with your steel supplier whether the steel is A36 or 50.

You say rebar 12" each direction--what diameter rebar? 3/8, 1/2, 3/4? Answer: I meant to say #4 rebars at 12 inches each way. #4 bars are ½” diameter – the reinforcements to be placed at mid height of the slab.

You say any I-beam would work, but then you show W8x10? Any particular reason? If any will work, would like to go with W6x9 to maintain as much head room as possible so folks won't bang their head on the beam accidentally. Answer: There is no particular reason of using W8x10 in the calculation besides it’s the I beam section that is commonly used. Once the modulus of elasticity S is determined, the only thing that matters is to make sure the beam selected must have an S greater than the S calculated. Since steel is priced based on weight, it is best to choose the beam that weigh the least. If headroom clearance is an issue, the beam with least depth is what is needed but it will cost more because it will weigh more. The other factor to consider is availability.

Lastly, to be clear--vertical support walls during pour-- you recommend 4 total at ~2 ft spacing in parallel with I-beam, correct? Answer: That is correct, also these supports or shoring needs to be left in place for at least 28 days before removing. Since concrete will reach its design strength after 28 days, the shoring must remain in place until concrete is strong enough to support the weight it is design to carry.