There is a version of Clockwork for the Eris, but I wont link to it; there have been several known Erises that have bricked that started with Clockwork installed as the default recovery.
You can install ROM Manager in your rooted ROM and have it flash Clockwork. This will create a file called update.zip, which you can flash from Amon_RA and then use safely (since the default recovery remains Amon_RA.) This is actually what I do with my Eris - I prefer Clockwork's Nandroid backups and wipe features over Amon_RA.
The Hero is not completely compatible with the Eris - for one thing, the Eris has a proximity sensor. There are a few ROMs that started as Hero CDMA builds that have been ported to the Eris, but any Hero ROMs that you try to flash will fail - Amon_RA checks for the Eris "desirec" code and will not flash a "heroc" ROM, as far as I know.