As far as your bio-load 1 inch per 5-10 gallons is the general rule. As far as swimming space needed, the individual fish needs to be addressed. For examply a tang will need so much feet of tank, where a clowns size is usually only considered for your bio-load. That's whare your extra sump water will be beneficial. Each individual fish or invert you consider should be appropriate for your 125 gallon, but the total inches of each fish combined (at full growth) is where you need to limit your choices. As long as you stay under 25 inches, total, and each species you chose is "happy" in a 125 gallon, you should be OK.
Is this making sence?? I'm trying to explain like 5 things at once and I'm getting all type tied. LOL