NVIDIA does not allow disparate graphics cards to be paired, plus they limit pairing to only high end GPUs in the hopes they sell more high end GPUs. A 1060 can't SLI pair with any other GPU. A 1080 Ti can only SLI with another 1080 Ti.
While very select few DirectX 12 titles have had support for disparate graphics cards to be used together, the trend so far is that you get very poor scaling, worse than had you just matched the cards in the first place. There is no gain from trying to pair up unmatched cards until somebody invents a better magic sauce, and that's not been done yet.