Few points:

*) When you place two legal entities in the same ledger it is assumed that they will be sharing the ledger options.
*) The Rounding Difference tracking account is specified at the ledger options and hence will be shared among all legal entities attached to that ledger.
*) You should specify this account if you are using subledgers. Oracle will give you an error if it encounters a subledger journal entry that is unbalanced because of rounding differences and an account is not specified.
*) If your transactions include different balancing segment values representing differenct companies/legal entities, then General Ledger and Subledger Accounting will automatically create a rounding differences account for each balancing segment value.

In your case if you have specified rounding difference account as: 900.300.901.80205.000

For company 900, account 900.300.901.80205.000 will be used.
For company 901, account 901.300.901.80205.000 will be used and so on.

If you wish to use 901.300.910.80205.000 for company 901 then this calls for separate ledger options setup, and hence a separate ledger.

*) Subledger Accounting does offer features for controlling the way accounting program deals with rounding differences.
You can explore further in this direction.