when Leaflet.markerCluster.addLayer() is called, an attempt is made immediately to check whether a single lat/long can be extracted and clustered:
if (!layer.getLatLng) {
this._nonPointGroup.addLayer(layer);
this.fire('layeradd', { layer: layer });
return this;
}
so you have several problems...
1. L.esri.featureLayer doesn't fetch any features at all until after it is added to the map and even then has to make asynchronous web requests to ask for features that intersect the area being drawn
2. even when these features are retrieved, they can't be pulled out of L.esri.featureLayer via the .getLatLng() method that plugin uses (above).
in the ArcGIS world, we tend to think of layers as entire collections of features with the same geometry type in a map. In Leaflet, a layer often refers to a single geometry (marker, polyline or polygon)
unfortunately, i don't have any helpful advice for conveniently managing and clustering data coming from several different services. it would take some work.