Re: tap rule for sub-panel
The smallest sub-panel I'm familair with is rated at 125 amps. If you use cable capable of the 100 amps, such as #2 AL, then you have no concern, except cost, which is where distance comes in.
How much lower than 100 amps are you considering? As long as you stay within 1/3 of the OCP, which means a 30-amp feeder is too small, tap rules may apply.
Tap rules say "240.21(B) Feeder Taps. Conductors shall be permitted to be tapped, without overcurrent protection at the tap, to a feeder as specified in 240.21(B)(1) through (5)."
Notice it says "Conductors shall be..." I don't know if that applies to lugs in a panel.