You need to have control over your own nameservers to setup rDNS on smallhost.com on your own. If the NS's are under the control of your provider, you need to contact them to put in a PTR (pointer) entry into their in-addr.arpa zone file.
If you control your own nameserver, you will just need to edit the nameserver zone for that domain and add a PTR entry to point to the doimain from their respective IP address.
you can either get full control over your rdns by telling the provider to delegate the rdns authority of those IPs to you (not common unless its an IRC server)
or you can email techsupport and ask them to set the rdns for your ip.