When the Church declares someone a Saint, it is the formal acknowledgment that the person is in heaven. However, there will be more people in people in the heaven than the church has declared as saints.
As to who we are allowed to pray to, I will defer to others. But since there has to be devotion to a person and recorded miracles resulting from seeking their intercession, before the church declares someone a saint, it logically follows that catholics aren’t limited the list of church approved saints to seek intercesion from.
FWIW, the good thief is a saint recoginzed by the church, also known as St. Dismas.
It occured to me that in an odd way, St. Dismas is the ultimate protestant saint. His sainthood is confirmed by the bible rather than church decree and thus from a protestant point of view inherenitly more reliable.