SLI can be done within same model....but different brand...for example an EVGA stock 980 could be SLId with an overclocked strix model 980...if both have same memory amount.
Different generations are generally not able to run in SLI. (it is possible sometimes with specially tricked out drivers but this is fringe stuff and voids warranties etc)
Also be careful on card sizes.. you wont be able to use some of the new High Bandwidth if the cards are bigger/smaller that each other, you will be stuck with using the old style ribbon connectors..