Question regarding Mod Soul Storage

Hypereia

So, from what I understand, you only receive 4 default slots to store mod souls. However, when you unlock 5 slots for formation, you don't gain an additional slot to store another mod soul.

Do you really not get more slots as you level, or as some form of bonus? Are you FORCED to spend gold, just to let all your partners in formation have a mod soul equipped?

I hope I'm mistaken, can someone clarify this?

Thanks.