If you want to change the way the spells work for your game, by all means go ahead.
Under the rules as written, Light and Darkness oppose each other, Glow I (Light) has an area of effect of a 60 foot sphere, Darkness II (Darkness) has an area of effect of a 60ft sphere, and when opposing spells (like Light and Darkness) occupy the same area one or more of them gets countered.
Keep in mind that the darkness from a spell is not
just the absence of light, but a sight sucking machine that prevents even darkvision II, which is completely independent of needing light from working. There is dark, then there is Dark. There is the blackness of space, then there is the hole in space from which no light can ever escape. Darkness from a magical effect doesn't just negate sources of light, it hunts down light and utterly destroys it in all of its form anywhere it can find it.
Okay after reading 112, the light from the TL3 NPC would counter the 1st level mage/2nd level character spell.
Assuming they both cast spells of equal level (such as Glow II vs Darkness II both being level 3), correct. But if following the original scenario of Glow I (level 0) vs Darkness II (level 3), the Darkness II would win out regardless of who cast which in this case.
I also think since the spell's levels are balanced power wise, a 3rd level spell is more powerful then a 1st level spell so that should be a factor also. Maybe I will house rule that the combined total of the spell level and caster level determines the strength.
No reason to house rule it, it is already the rules as written.
"When two opposing spells occupy the same Area, the spell with the lower Casting Level + Spell Level is countered. When these values are equal, both spells are countered."
If a PC Mage 1/Whatever 2 somehow casts Darkness II and TL 3 Wizard casts Glow I, the PC's spell will counter the NPCs Glow I. (PC Caster Level 1 + Spell Level 3 > NPC Caster Level 3 + Spell Level 0).
If a PC Mage 1/Whatever 3 somehow casts Darkness II and TL 4 Wizard casts Glow I, both spells will be countered. (PC Caster Level 1 + Spell Level 3 = NPC Caster Level 4 + Spell Level 0).
If a PC Mage 1/Whatever 4 somehow casts Darkness II and TL 5 Wizard casts Glow I, the NPC's spell will counter the PCs Darkness II. (PC Caster Level 1 + Spell Level 3 < NPC Caster Level 5 + Spell Level 0).