That was a weird story in which a 30 year old fuse has blown up, nothing more.
Of course you have to take care to insert the cartridges the right way, always shut up the computer, when changing or entering a cartridge. So nothing more to worry about when connecting a slot expander to your MSX.
The PSU of a computer is designed to sustain certain load. If the load exceeds the capacity, the PSU may fail. Some PSUs have fuses to prevent catastrophic damage in the event of short circuit or excessive load. When you have 4 cartridges that require power used with the computer with only 2 cartridge slots, you have a chance for overloading. Normally the slot expander should supply the power to the inserted cartridges to help the computer's PSU. If that does not happen, all power will be drawn from the computer. As said above this may lead to PSU's overheating or failure. In the well designed PSU the fuse will save the day. Please correct me if I am wrong.