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