If the charger works for NIMH, then it will work for Nicad. Not necessarily the other way around.

You are right that 50ma is 1/4 of 200ma or .2 amps.

To figure out initial charge time, take the capacity of the battery, multiply by 1.4 (for losses) and divide by charge rate.

So, 2.3x1.4/.05=64.4 hours