Testing individual battery cell with load applied (how many amps should I apply?)

You perhaps was talking about module (7.2V) not cell (6 cells in every module, 28 modules in your Prius).
If you are going to compare your results with someone's else results - you both use the same method, same currents.
It is important how modules behave under the same load in the pack.
Capacity measured at high currents will differ from measurements taken at lower currents.
6A is OK.
My tools are only for professional use and not for individuals.
But you can get an idea of how battery behaves under load from my videos here: www.hybrids.co.nz - YouTube

 

This is an old thread, but the question is still a good one.

The official rated capacity of Prius NiMH modules is 6.5Ah (aka 6,500mAh).
The meaning of this is that applying a 6.5A load will fully discharge the module in 1 hour.

In contrast, NiMH battery manufacturers generally apply a C/10 load current to rate the mAh capacity.
For Prius NiMH modules C/10 is 650mA (aka 0.65A).

As stated by kiwi, higher load currents will reduce the measured mAh capacity. Therefore testing at 6A will never give a result of 6.5Ah. However this does permit completing a test in less then 1 hour. 6A is OK when comparing a group of used modules to each other as long as all are tested at the same discharge rate.

Because I'm a retired Prius owner with time to spare, I do my module testing with a 1A load.