Hi,
I have a female Hardwicki living perfectly with a male Mali (and a female Mali). I don't know about Nigerian personalities... The best way to introduce a new Uro to another one, is to give them a bath together (after quarantine, of course) and then to introduce both to a brand new cage, so there is no real "territory". Also, watch them for the first couple of days - sometimes fights don't happen until after awhile. And keep an eye on their weight, as one can become dominant and keep the other from eating. Hope this helps 
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~Sarah
1.1 Mali Uromastyx (Draco and Pumpkin)
0.1 Indian Uromastyx (Cheddar)
1.0 Russian Tortoise (Pookie)
8.9.22 Millipedes (9 different species)