Provided they are written properly the number of plugins should have little effect on the server. Also, I would strongly advise against rewriting all plugins in Skript since a well optimised plugin will always perform better than even the most optimised script. As well as that, ClearLagg should only be used if the number of entities is actually a problem since it can be frustrating for players, and so if that is a problem it's still a better idea to just reduce the maximum age of items, making it more consistent and predicatable for players.
@FabricioSouza there is no one way to reduce lag on a server, the cause of lag will always vary from server to server and so without a timing report we can only give you general rules to reduce lag