Adding another will be fine. the larger one may pick at it for the first few minutes, but that sould be it. I have added four 1 inch uaru in with three 6-7 inch uaru once and they were all fine.
I just added two large 8 inch uaru in a tank with two other uaru the same size, and they liplocked and nipped at eachother for around 5 minutes, but that was it.
I doubt you'll have any issues adding another either.