My experience with NiCds is that if you don't use them, you lose them. They tend to grow crystals and one cell will short out,killing the pack. I have had much better luck with NiMH.
NiCd and NiMH charge profiles (except for fast charge, specifically voltage and temperature detection of end-of-charge) are very similar, so in many cases you can sub one for the other. If the charger is a typical 0.1C charger, you should be fine. The good quality NiMH calls are spec'd to tolerate 0.1C overcharge for a long time. Even if you're using a fast charger, it may be quite simple to modify it to support NiMH.
I have replaced NiCds with NiMH and almost without exception, the NiMH cells have much higher capacity than the NiCds they are replacing. Watch out for output current rating, though. For a flash, you want to spend the extra money on high-current output rated cells.
NiCds are getting hard to find, due to the Cadmium being hazardous waste. Googling will bring up a couple of places that sell all kinds of raw cells, and will even spot-weld them into packs for you.
I should also mention that there are chargers available, designed for the hobby remote control market, that will charge multi-cell packs. This may be simpler than modifying a fast charger. They run about $50, and they're Chinese, so the first one you get might be bad, but we have two we have been using in our lab for a couple of years without any problems (many manufacturers, they all look like this one, though):
http://www.all-battery.com/Tenergytb6ac-charger-01322.aspx
Source: working EE - I design portable equipment with NiMH and LiIon cells.