1: i have only one 7815 server in site1 and cluster of pub and sub in site2, i want to config an intercluster trunk between the 7815 in site1 and cluster in site2 is it applicable as 7815 is one server not a cluster or it doesn't matter?
2: same thing but now if i have one server 7815 in site1 and one server 7815 or 7825 in site2 and i want to config intercluster trunk between 7815 in site1 and 7815 or 7825 in site2 is it applicable as now they are one server in each site not cluster to cluster or is it also does not matter?
Both of your scenarios are still valid with one or more servers. The word "cluster" is a little misleading because in some instances like you have noted the "cluster" consists of a single server. An ICT can go between single server clusters as well :)
Hope this makes sense!