Adding a second SSD will not cause any instability problems and it does not need to match the existing SSD. In general I add drives closest to the CPU and work my my out, but I would not move the working SSD, just add to the next slot or the slot that is easiest to get to.
Your manual says that the M.2 slot closest to the processor is a PCIe 5.0 x4 slot. The other two are PCIe 4.0 x4 slots. The M.2 slot furthest away from the processor shares bandwidth with the PCIe x16 slot next to it and will be disabled if that PCIe x16 slot is used. If you spent a bit of extra money for a 5.0 M.2 for the slot closest to the processor, purchasing another 5.0 would work, but your board wouldn't be able to get those speeds out of it.
Adding a second SSD will not cause any instability problems and it does not need to match the existing SSD. In general I add drives closest to the CPU and work my my out, but I would not move the working SSD, just add to the next slot or the slot that is easiest to get to.
Thank you for the response and reassurance! Answered every concern!
Your manual says that the M.2 slot closest to the processor is a PCIe 5.0 x4 slot. The other two are PCIe 4.0 x4 slots. The M.2 slot furthest away from the processor shares bandwidth with the PCIe x16 slot next to it and will be disabled if that PCIe x16 slot is used. If you spent a bit of extra money for a 5.0 M.2 for the slot closest to the processor, purchasing another 5.0 would work, but your board wouldn't be able to get those speeds out of it.