For a 125 I would use a mag 12 and have Use 1.5 " drain lines. If mag 350 means 350 gph
then your only going to get 1 x turnover after headloss. if it is a ph your referring to that is still only 2.5 x turnover...
As for getting a pump to match the drain lines capacity, this would only be necessary if your tank is drilled on the bottom and you do not have an overflow box and stand pipe which would be a really bad idea because your tank inhabitants would go down the drain.
The drains will pull out the amount of water you allow them to meaning the amount of water that drains out is a function of how much you are pumping back in. I would not recommend trying to match the drain lines capacity with the return pump bc if the drain becomes even slightly obstructed water will spill over the DT.
I have two 1" drain lines which can sustain up to something like 1200 gph
each! I also use a mag 7 on a 75 and am getting around 3-400 gph
back into the tank as I have a t feeding the refugium an there is 3 ft of headloss. There is no possible (well very improbable lol) way for me to get a flood out of either the sump or DT.
I have power heads to supplement Flow in the DT.