do .3 from promos stack so if I have 5 t1 spheres it would increase by 1.5?
Yes, but you'd be better off just taking the promotion you want in the first place.
The only time it makes
any difference is when the promotion is free, from mana or Govannon or something. Any time you have the choice between
either a promotion that you want
or a promotion that boosts your xp rate, it is always better to take the useful promotion.
Adepts might be a semi-exception. They need a bunch of filler promotions to get to level 4
and they are in the narrow band where passive training has an effect
and the first level spells are often the best promotions available anyway. Even then: Spell Extension, Mobility, Combat, (March)...
Speaking of Govannon, the xp rate boost is not for arcane units only. So having Govannon give low xp units; workers, melee, disciples, anything; a bunch of first level spells boosts their training as well.
