It is possible to set slot length and epoch size when starting network from Haskell via PlutipConfig -- extraConfig :: ExtraConfig holds corresponding fields.
For setting parameters when launching local-cluster executable see --slot-len and --epoch-size options in the local-cluster documentation.
It is possible to change some settings of a local network that Plutip starts. By default Plutip uses node config, genesis files and etc. from the cluster-data directory.
It is not advised to change anything in cluster-data. Better way would be to copy cluster-data to a desired location, change what is needed and then point Plutip to this custom directory by modifying the clusterDataDir field of a PlutipConfig parameter to withConfiguredCluster or startCluster.