Doubts about usability testing

For example, if we have some mockups and we want to test it, using a lab test, to check if they are ok to implement we should:

• Do only a formative test to identify issues?

• Or we can also do a summative with the mockups to verify errros, eficiency and effectiveness? Or this summative test should only be after implement? Also it seems that its ok to test with just 5 users to identify problems, that is to make formative tests, but for summative (to measure errors, efficiency and so on) tests, with just 5 users can be enough also?

• Note, but if we should only do a formative test with the mockups, a formative test should include what? What can we test to identify problems? We can use summative metrics in the formative test?