A fix for
MB-34280 was put into 7.0.0 and 6.6.5 which limited the max connection length to 200 characters. Fixes for the companion ns_server issue, MB-35881, were put into 6.6.5 and 7.0.2 that limited the length of the names of DCP replication connections to 200 characters.
Unfortunately, there's a bug in the ns_server fixes in that in mixed mode clusters, up-version nodes refer to the truncated name of replication connections, but down-version nodes refer to the untruncated name and rebalance stops when ns_server gathers stats against a connection that doesn't exist. Even if the bug in ns_server weren't there there would still be a problem in that the up-version nodes would not accept connections of more than 200 characters but the down-version nodes do.