Ok, according to the internet, summoners can only cast Shiva as a level 2 summoner/summon ability (at the lowest).

My Reina is able to cast Shiva at level 1 summoner...

However, she couldn't before... but now that Galuf is level 2 summoner, this has also enabled Reina to cast Shiva at level 1. Is this supposed to happen?

Similarly, all of my characters can cast both black and white magic level 2 spells, even though some characters are at level 1 in those classes!

Does reaching a certain class level unlock stuff for EVERYONE then? (As I thought it was just for each character individually...)

I am playing the PS1 Port btw, from the PS Store.