Given that your info is correct (6S NiMH blocks with 7.2 V rated voltage and 6.5 Ah capacity) here my proposal:
I don't know if discharging them first will bring a big advantage. If you want to do it, I'd not overstress them, maybe around 1 A for discharging is OK, be sure to take care not to let the voltage accidentality below 7.2 V!
For a 6S module I would use of 8.4V (be precise) constant voltage charge with a current limit around 0.4 A (current should be well below 1/10C to be on the safe side (for your 6.5Ah cells 1/10C would be 0.65A). This charging is so slow that even a little overcharging should not damage the cells within a reasonable time.
A lab power supply with CV/CC mode and voltage/current display would be great, but you can go simple with a multimeter and any stable and adjustable supply as well.
To detect end-of-charge there are several options. A timer should be the least - additionally looking at the current (it reduces notably when near full) or the temperature (it rises when near full, but maybe hard to detect at this little current) do work.
Charging with the mentioned 8.4 V and 1/10C is said to take 12-15 h. If you choose the lower current, prolong the timer accordingly (~ 20 h).
Of course a multi-cell NiMH-charger with delta-V detection would be better and could charge quicker, but it seems you don't have one. Also the -dV comes after the +dT so looking at temperature seems more clever to me.
There are 28 modules - will take you a lot of time :)