Do you want to add the new device for High-Availability on the network with the MX100? Then it will not work as both units have to be the same model.
But if you want to add an additional network (like a new branch office) that connects via AutoVPN, then you can take any model you want. This is quite common. For example one customer has the MX250 on the headquarter and a mix of MX67/MX68/MX84 in the branches.
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