There is a bug with the Gelatonius Armor.
The baby pink slime it spawns seems to count against your minion limit. Don't know if that's deliberate or not, but it is sometimes possible to summon another minion when you're already at maximum minions, and then what happens is the pink slime despawns, and then the armor seems to try to rapidly re-summon the slime every frame, and fail to do so. When this happens, you see the baby pink slime's buff icon rapidly flashing.
The issue seems harmless, but is somewhat annoying.
And as far as I know, none of the vanilla armors summon a minion as a set bonus, so whether or not it should count against your minion limit is an open question. The closest thing are the Forbidden Armor and Stardust Armor, which summon a sentry-style minion. AFAIK, they do not count against the sentry limit or the minion limit.