Jose, for the common plug and perf completion method, you are correct. The plug and guns are run in the hole together. There are switches between the plug and each cluster of guns. The switches allow the operator to detonate each charge separately by applying either a positive or negative voltage to the wireline.
In all of the completions that I have seen, each tubular would need to be rated for at least as much pressure as is likely be encountered plus a safety factor.
Is you customer planning to treat down the casing or through another tubing string?
I have seen scenarios where you have 7" production casing and a 4-1/2" liner where the liner has a polished bore receptacle (PBR) liner hanger top. In this case a seal assembly stinger is run into the well on tubing and stabbed into the liner hanger. In this instance, the 7" production casing would likely not have to be rated to full treating pressure since it should not experience full treating pressure. Though, the liner, liner hanger, PBR, seal assembly, tubing, and wellhead components would need to be rated for expected treating pressure plus a safety factor.