10g is awesome. 10g ASM is really fantastic!

No compatability problem. In fact if you want to use Grid Control (10g Enterprise Manager) the repository cannot sit on a 10g database .. and since 10g doesn't talk to 8.1.7.3 or earlier versions, you're left with 8.1.7.4 thru 9.2.0.5

However if you only run a single 10g database on that server, you can use "Database Control" to manage it. Just remember, one server, one instance, only one "Database control". Add another instance and you'll have to pick which one to manage. There is an alternative though... you can stop the dbconsole, reset your ORACLE_SID, and startup the other instance's dbconsole. It's a mess.

Best of luck. 10g release 2 is due out in June (Ken Jacobs interview on SearchOracle.com) We're hoping to see a few of these things cleared up.