A.
Matters referring to Radioactive Material with other sub hazards starts from DGR 3.10.2 and is further defined in 3.10.3 and SP A130 where it is explained that Radioactive Material having other hazardous properties must always be classified in Class 7 and the subsidiary hazard must also be identified other than for excepted packages of Radioactive Material where the other hazardous properties take precedence. There is an example in DGR Section 8 Figure 8.1.O Example 11 with a footnote explanation on the labels.

As for the requirement of labels, please refer to 10.3.10.1 and 10.7.3.2.
In answer to your example about a Class 8 sub hazard label, there are 3 cases.

(1) Radioactive material is of normal activity level and Class 8 sub haard is EQ level – there is no need to identify the sub hazard, and no Class 8 label is required.
(2) Radioactive material is excepted package and Class 8 sub hazard is normal quantity - Shipment is declared as Class 8. This is illustrated in Figure 8.1.O Example 11.
(3) Radioactive material is excepted package and Class 8 sub hazard is also EQ level – Shipment is declared as Class 8 EQ.