I am using my ETS EC 150/5 with both sizes of pad (festool) very happily. . .
I purchased the ETS EC 150/5 and simply modified the 125mm sanding pad to fit with a multitool/dremel. It works 100% great.
Having owned the 125mm sander and used the 150m pads on it for years I realised that the round mount drive shaft, with flat sides, on the 125mm sander was more than enough to secure the 150mm pad without the extra 'fins' that the 150mm sanders have on their drive shaft.
So the modification/adjustment to the 125mm pad, to fit the 150mm sanders doesn't even need to be that neat or accurate, in relation to the fins. You simply need to remove enough plastic from the 125mm pad to allow the 'fins' on the 150m sander shaft to fit into/onto the pad. The main shaft with it's flat edges, is in reality enough to hold the pad tight in position.
And the pad design has the metal plate/disc inside, that the drive shaft seats onto, so knowing how deep to remove the plastic is easy, you just remove the plastic until you hit the metal plate.
IF you are wearing through pads every month or two then this is obviously not a great solution, but if like me you just want the occasional use of the 125mm pad, this is a fantastic solution. And honestly, I've never used a sanding pad that lasts as long as a festool pad.
NOTE: I did mine fairly accurately, but not in a way any engineer or machinist would call actuate

It took me about 10-15min to modify and it's worked fine for months.