If using a subystem, do i still use the Replication System of UE4?

Using Steam for example.

Yes, OnlineSubsystems are just to support gaming social network (like PSN, XboxLive or SteamWorks) features support, like friend lists, server lists, achivments, leaderboards etc. Non of those networks enforce you to any game protocol, you are free on that matter.

Ok, thank you Shadowriver. (: