Generally, it's safe (at the discharge side) to replace NiCd with NiMH, although the reverse is not necessarily true (my digital camera says that it's OK to use alkaline or NiMH, but not NiCD). As for charging, most (all that I've seen, but there's bound to be an oddball out there) NiMH chargers can safely charge NiCd, but the reverse is not true. NiCd can handle a constant-current charge, and a "trickle" doesn't harm them so long as it's below a certain rate (IIRC, 0.1 of the amp/hour rating). NiMH doesn't like trickle charges - from what I've heard, it likes to guzzle electrons, then get cut off when one parameter (can't recall if it's cell voltage, or rate of voltage change) hits a certain value. Since the cheap way to do a NiCd charger is constant current at a "safe" trickle rate, I'd invest in a separate charger if you go with NiMH.
As for the rechargeable pack being a single unit, that sounds like a convenience issue - only 1 piece to handle instead of 2.
You didn't ask, but high-drain equipment (typically photoflashes) designed for alkalines can be damaged if you use NiCd. This is due to the extremely low internal resistance of NiCd (a 12V string of "D" size can start a car, but this'll run it flat in a few seconds and probably damage the cells) - under heavy drain, the low internal resistance can cause them to have a higher terminal voltage than alkalines. In cheap flash units, the inverter isn't designed to handle the higher voltage, and burns out.
