The fuse is a safety measure, If you have an overcurrent draw on a LiPo OR on an NiMH battery, the battery will burn. Difference is the LiPo is going to burn a LOT hotter with fire spewing out the side. There are a LOT of AEG's that don't have fuses, that doesn't mean they don't need them.
Things that cause over current draw are shorts in the wiring, your motor jamming and too much torque on the motor (having a powerful spring and the wrong gear ratio)
It's a good idea to have a fuse with NiMH as well, but LiPo's are more dangerous and it's a REALLY good idea to use a fuse, especially if you have a systema magnum motor.
But you need to get the right fuse, using a 30A fuse with a LiPo that can only supply 25A is useless.
There's dozens of configurations, shapes, sizes and connectors for NiMH batteries. I suggest getting deans connectors since they're the best for airsoft.
Check ehobby asia, they have a large selection of airsoft specific NiMH batteries.