SLI is rarely supported well on newer titles, so I'd suggest a single, stronger GPU - however you can achieve that.
On modern cards, SLI is only available on >GTX 1060's (so 1070, 1080 or 1080TI's)
You cannot "SLI" different GPUs. They must have the same Graphics chip (so 1070 et).
You can use Explicit Multi Adapter within DX12 to utilize the GPU horsepower of differing cards - but support is seriously lacking (only utilized by Ashes of the Singularity AFAIK).
What;s your budget? And what have you purchased already? (if anything).