The OP seemed to have started another 3 threads at least about this. Why not keep it all in one. I understand there might be a language issue since the OP is from Rio, and I don't speak portuguese, if that is the case please try asking something specific in this thread and I will do my best to help.
Simple answer to all you have asked:Yes.
Yes you can use a rapier in your main hand and have a hand crossbow in your off hand.
If you do not have the crossbow expert feat that gives you the option to attack with either on your turn as your action, but not both.
If you do have the crossbow expert feat, you can attack with your rapier and then with a bonus action attack with your hand crossbow. Yes you do add dex modifier to the damage of both attacks.
If you do have the crossbow expert feat, you can attack with your hand crossbow, and then again with your hand crossbow as a bonus action. Yes you do add dex modifier to the damage of both attacks.
You do NOT want or need either the duel wielder feat or the two-weapon fighting style class feature.