If you create an additional snmp collection in datacollection-config.xml,
arbitrary indexes do not work in not default-collections.

I added a package in collectd-configuration.xml:
(there is a fancy filter supposed to be set, I removed it for testing with no difference)

------------------------------------------------------------------------ <package name="fsc">
<filter>IPADDR != '0.0.0.0'</filter>
<include-range begin="1.1.1.1" end="254.254.254.254"/>
<service name="SNMP" interval="300000" user-defined="false" status="on">
<parameter key="collection" value="fsc"/>
<parameter key="thresholding-enabled" value="true"/>
</service>
</package>

------------------------------------------------------------------------
created a collection named fsc in datacollection-config.xml:

------------------------------------------------------------------------ <snmp-collection name="fsc" maxVarsPerPdu="10" snmpStorageFlag="select">
<rrd step="300">
<rra>RRA:AVERAGE:0.5:1:2016</rra>
<rra>RRA:AVERAGE:0.5:12:1488</rra>
<rra>RRA:AVERAGE:0.5:288:366</rra>
<rra>RRA:MAX:0.5:288:366</rra>
<rra>RRA:MIN:0.5:288:366</rra>
</rrd>

<resourceType name="fsctemp" label="FSC ServerView Temperature Sensors"
resourceLabel="${fsctempSens} (index ${index})">
<persistenceSelectorStrategy class="org.opennms.netmgt.collectd.PersistAllSelectorStrategy"/>
<storageStrategy class="org.opennms.netmgt.dao.support.IndexStorageStrategy"/>
</resourceType>

<resourceType name="fanCabinetId" label="FSC ServerView Fan Sensors"
resourceLabel="${fanDesignation} (index ${index})">
<persistenceSelectorStrategy class="org.opennms.netmgt.collectd.PersistAllSelectorStrategy"/>
<storageStrategy class="org.opennms.netmgt.dao.support.IndexStorageStrategy"/>
</resourceType>

<groups>
<group name="fsc-sys" ifType="ignore">
<mibObj oid=".1.3.6.1.4.1.231.2.10.2.2.5.10.3.1.3" instance="0" alias="fscSerNum" type="string" />
<mibObj oid=".1.3.6.1.4.1.231.2.10.2.2.5.10.3.1.4" instance="0" alias="fscModel" type="string" />
<mibObj oid=".1.3.6.1.4.1.231.2.10.2.1.4" instance="0" alias="fscOSVer" type="string" />
</group>
<group name="fsc-temp" ifType="all">
<mibObj oid=".1.3.6.1.4.1.231.2.10.2.2.5.2.1.1.6" instance="fsctemp" alias="fsctempUpWarn" type="Gauge" />
<mibObj oid=".1.3.6.1.4.1.231.2.10.2.2.5.2.1.1.8" instance="fsctemp" alias="fsctempUpCrit" type="Gauge" />
<mibObj oid=".1.3.6.1.4.1.231.2.10.2.2.5.2.1.1.11" instance="fsctemp" alias="fsctempCurVal" type="Gauge" />
<mibObj oid=".1.3.6.1.4.1.231.2.10.2.2.5.2.1.1.13" instance="fsctemp" alias="fsctempSens" type="string" />
</group>
<group name="fsc-fan" ifType="all">
<mibObj oid=".1.3.6.1.4.1.231.2.10.2.2.5.2.2.1.8" instance="fanCabinetId" alias="fanCurrentSpeed" type="Gauge" />
<mibObj oid=".1.3.6.1.4.1.231.2.10.2.2.5.2.2.1.9" instance="fanCabinetId" alias="fanNomMaxSpeed" type="INTEGER" />
<mibObj oid=".1.3.6.1.4.1.231.2.10.2.2.5.2.2.1.10" instance="fanCabinetId" alias="fanCurrMaxSpeed" type="Gauge" />
<mibObj oid=".1.3.6.1.4.1.231.2.10.2.2.5.2.2.1.11" instance="fanCabinetId" alias="fanMaxSpeedCrit" type="INTEGER" />
<mibObj oid=".1.3.6.1.4.1.231.2.10.2.2.5.2.2.1.12" instance="fanCabinetId" alias="fanMinSpeed" type="INTEGER" />
<mibObj oid=".1.3.6.1.4.1.231.2.10.2.2.5.2.2.1.14" instance="fanCabinetId" alias="fanFailReaction" type="INTEGER" />
<mibObj oid=".1.3.6.1.4.1.231.2.10.2.2.5.2.2.1.16" instance="fanCabinetId" alias="fanDesignation" type="String" />
<mibObj oid=".1.3.6.1.4.1.231.2.10.2.2.5.2.2.1.17" instance="fanCabinetId" alias="fanIdentification" type="String" />
</group>
</groups>
<systems>
<systemDef name = "Enterprise">
<sysoidMask>.1.3.6.1.4.1.</sysoidMask>
<collect>
<includeGroup>fsc-sys</includeGroup>
<includeGroup>fsc-temp</includeGroup>
<includeGroup>fsc-fan</includeGroup>
</collect>
</systemDef>
</systems>
</snmp-collection>

------------------------------------------------------------------------

Result: no fsc-fan / fsc-temp values collected

fsc-sys - values (fixed index) however are collected.

If I put the resourceType, group and includeGroup statements into default collection, data is collected an written to disk.