As a Siren you can slag enemies via phase-lock, use electricity/transfusion grenades with cluster/homing and use a sniper rifle as you see fit Slag in solo is a mainly a "slag em, then frag em" kind of deal. You can't use it with weapons imo (in fact, slag on weapons is imo the worst elemental choice.
If you don't have a skill that slags, using slag transfusion homing cluster grenades is a good idea
As for where it says 4 ammo.. nowhere! ;p I would call that a bug..
I have to kind of agree though, gun combinations feel sorta limited now. And there is also the little issue that frost elements are missing. And Eridium weapons are extremely rare...