900 is pretty much the same size as the 1200, so won't make much difference in your tank. Does draw less than half the power for more than 2/3 the flow, so that seems like a good trade-off. I have two mj1200s in my tank for flow, and am considering swapping them out for 900s so they'll last longer on back-up power.
The remora creates back-pressure, so the turnover is less than the 296 g/h (or so) on the 1200 spec. If you can get the regular Remora to fit on/in your tank, I don't see a problem with it.