Can one of Windows failover cluster nodes run on Linux?
As SQL Server is available for Linux can I configure Clustering on Linux nodes?


You can configure Corosync and pacemaker for Linux. Good luck.


  I mean mixed with Windows - some nodes are on Linux, others on Windows in the same cluster?


In the same cluster, no. Someone will argue with me and say, "Well, Sean, you can do read-scale availability groups like this!" and that may be true, however it isn't a cluster.

The only way to do this would be to have two different clusters, spanned by a distributed availability group. If you want to make life complicated then this would be the setup to do it.