It depends what you wanna do. A seniority list is just about your time in the company. It usually comes in effect with leave, days off, promotion etc. It has very little to do with experience.
So even when a DEC has less experience it does not effect the seniority list.
In the cockpit the DEC is still the CPT and for this he is in charge.
On the way back home the FO might get the last jumpseat because of his seniority and the CPT has to wait the next flight
But usually companies with seniority list do not hire DEC with less experience than their senior FOs.