Hi,
Very late answer and you probably has this figured out by now. Anyway, I put it here for reference.
You can configure all the other search heads to only run ad hoc searches.
In server.conf add the following:
[shclustering]
adhoc_searchhead = true
Ref: https://docs.splunk.com/Documentation/Splunk/6.5.1/DistSearch/Adhocclustermember
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