SCCs only make sense for directed graphs where edges have direction. In an undirected graph, if you can reach node from node , you can always reach from by reversing the path. Every connected component in an undirected graph is trivially "strongly connected".
For undirected graphs, use simple DFS or BFS to find connected components in time. No need for SCC algorithms like Kosaraju or Tarjan.
Space complexity is for the data structures used.