To use rpm based flow sensors with the Aquaero you need to know the number of rpms equals a litre. All the AC flow sensors come with these values for each sensor but other brands don't. You would need to work it out yourself or hope someone else already has.
It is also the case that the mechanical flow sensors from AC use a two pulse per rpm system whereas other after market models use a single pulse per rpm system to be compatible with ordinary fan control headers.
The result is that pulse per litre figures for other flow meters are very low in comparison and don't play well with the Aquaero.
I highly recommend using the proper AC system models.

The temp sensors from most common brands all use the same NTC film parts so work just fine.

Yes, no problem to do that.

You can only adjust the impulse rate per liter. The accuracy mainly depends on the sensor itself and has not much to do with the aquaero.

The impulse per litre figure is the conversion rate. The aquaero converts the rpm signal it gets to a flow rate. If the calibration figure for that flow meter is 86 and the sensor is reporting 172 then the Aquaero displays 2LPM, 256 means 3 LPM and so on.

In this way the accuracy of the system is all in how accurate that calibration number is. As you say, the calibration number is affected by tube diameter and fittings used so to get more than a representational number you would need to use a high accuracy calibrated flow meter to test the bitspower model and your tube/fittings against for an accurate calibration at one flow rate. Error will be introduced as the flow rate increases or decreases from that flow rate.
For a really accurate system the AC pressure based meters allow you to define a flow/pressure curve so that the meter is accurate through a range of flow.

For most users a basic flow representation is all the accuracy that is needed.
There is further reading available on flow sensors available at Extremerigs testing of the MPS 400 flow meter, The community water cooling testing thread at OCN, and Martins liquid labs review of the Aquacomp mechanical high flow meter.

http://www.xtremerigs.net/2013/04/30/aqu…00-flow-sensor/
http://www.overclock.net/t/1501978/ocn-c…ing-test-thread
https://martinsliquidlab.wordpress.com/2…w-sensor-meter/

Again, I highly recommend just using the meters that are designed for the Aquaero. They are far higher quality than the flimsy Bitspower spinners that have a habit of leaking.