As per the subject, if one mixes MCBs and/or RCBOs of different makes in a consumer unit is it likely to stop it conforming to IEE rules?
Also I presume the new (18th edition) rules regarding flammability of CUs would only apply if one replaces a CU and are not retrospective. Adding or changing MCBs to RCBOs in an existing CU wouldn't require that the CU be upgraded to 'non flammable'.