When reusing scan rules from a similar sub role, typical approaches include:
Replace enterprise OID part. In most cases, you can adapt existing rules to detect similar devices of a different brand simply by changing the enterprise part of the MIB OID.
Re-use SNMP module identifiers. OIDs that identify device types (NICs, printers, UPS's) or software protocol stacks (HTTP, OSPF, IP), can be re-used if you are extending a role to find similar devices of a different brand or vendor.
Check logic within and between rule groups. When you copy a Default role, understand that by design they are generalized and use quick and simple 'match-any' evaluation (any one rule in a group can nominate the role). When you add scan rules that are more specific (let's say for a specific device brand), you will need to adjust your rule groups.