Yep definately not a problem in series. Only, if the capacities are vastly different you'll end up over discharging the smaller pack. Likewise, if you charge two packs in series, if they have different discharge levels you'll end up over charging one of the packs.

Keeping your wires short will reduce the resistance thus increasing power.

Connecting NiCd's and NiMH in parallel is a different matter and definately a bad thing to do when charging them.