You can pray to every deceased person. especially if they were good and saintly Catholics. This is a wholesome practice always being done by pious Catholics.
That's good to know, but they must have been Catholics, right?
My grandfather had six priests at his funeral, I always prayed to him as a boy, sometimes I still do, but I was uncertain of whether it was okay or not since I became a Trad.
Of course, they should have been in good standing with the Church.
Saintly people were immediately prayed to after their death. When a miracle follows, this is one of the necessary conditions for a formal beatification. Remember, beatifications/canonizations were not a formal procedure before the High and Late Middle Ages.
If you read the life of Saints or saintly people, it is always related how the faithful prayed to them directly after their departure. Relics are often distributed before the actual beatification takes place.
A good example for the sedevacantist side would be Padre Pio who is considered to be a Saint by most trads, even though his beatification might have been invalid.