So, if I equipped Horus LV6 with Rainbow Veil and during the Battle Phase, attacked a pumped up Dark Crusader, you're saying that it won't negate the ATK increase of Dark Crusader during the Battle Phase? Horus LV6 can't be equipped with Rainbow Veil?
Horus the Black Flame Dragon LV6
CAN be equipped with Equip Spell Cards. Equipping a card to a monster is
NOT the same as affecting the monster with a Spell Card.
Equip Spell Cards like Axe of Despair will, however, NOT boost the ATK of Horus LV6, because Horus is immune to Spell Cards. But Axe of Despair can still be equipped to Horus LV6.
There are some Equip Spell Cards that do
not affect the equipped monster. Rainbow Veil is one of those. All it requires is that the equipped monster battles something - this is NOT the same as affected the equipped monster. When the equipped monster does battle something, Rainbow Veil's effect will become active, and will negate the effects of the opposing monster. Note that it is still the effect of the Equip Spell Card.
So you CAN equip Horus LV6 with Rainbow Veil, and Rainbow Veil WILL negate the opposing monster's effects as normal. The equipped monster being immune to Spells has nothing to do with Rainbow Veil.
Union monsters, while equipped, have one or more effects. The first is usually some kind of ATK boost, which will be useless when equipped to a Spell-immune monster. If not, then it's something like "when the equipped monster attacks your opponent directly, your opponent discards 1 card". In that case, that effect will NOT be affected/changed/negated, because it's not affecting the equipped monster. Do you see why?
The second effect is a characteristic of Unions, and says that it will be destroyed instead of the equipped monster if it is destroyed in battle (some are even more general, and protect from destruction in general). This does NOT affect the equipped monster. It will work without hindrance.
• If "Horus the Black Flame Dragon LV6" is equipped with "Metallizing Parasite - Lunatite", and would be destroyed as a result of battle, "Metallizing Parasite - Lunatite" is destroyed instead (this is not negated by "Horus the Black Flame Dragon LV6"'s effect).
The key point of Spell-immune monsters is that they are simply not affected by their effects. They CAN be affected by their costs (being Tributed, usually, e.g. Mystik Wok). Spell-immune monsters do NOT prevent Equip Spell Cards from being activated; they just won't be affected by them in the end. Any effects those Equip Spell Cards have that do NOT affect the equipped monster will work as normal.