In a normal match I'd say no. The calling of EotG is treated as a summon, so IMO it suffers the limitations on number, and it's up to your meta to decide if you call the min or the max number of models (for example, 5 or 10 knights). For matched plays who knows? maybe yes, you could try to justify a 40 skink unit with the fact that you must still pay the price of the called unit.
So in matched play, say I build 1k point list and field everything including a Slann. Can I then summon anything? Or do I need to say make an 800 point list on the table and 200 points for summoning things and once that 200 is gone I can't summon anymore?
Exactly. If you field all the 1000 pts you don’t summon . If you keep a 200 pts reserve you use it and then it's over.