Certainly you should understand the buoyancy characteristics of the immersion suit you wear. You really should do this by reading it's instructions, and then going into the water wearing it. If you can wear a lifejacket over it, and you're confident that doing that will not make you too buoyant to swim down to exit, or that the need to do that could not occur, then I would certainly wear the lifejcaket! I have three immersion suits, one dry, and two not dry. Both of the not dry suits are also legal lifejackets, so wearing one over would be redundant. My dry immersion suit is not a legal lifejacket, though is very buoyant, both because of the insulating foam, and trapped air. That suit has built in gloves which are only moderately suitable for manipulating the hardware of a lifejacket, and I'm not confident that the combination of suit and lifejacket would make it easy to find and pull the inflation toggle on that type of lifejacket.
So, ultimately, know the requirements,both legally and practically, understand how you are going to comply with them, including understanding how your equipment works - by actually using it, and plan for the extremes of the situation you could experience: Maybe having to swim down, in which case, don't be too buoyant, having to use your hands to manipulate latches and buckles, and what you will do if you're injured, or your primary plan fails. So practice!
And, I cannot recommend highly enough, taking an underwater egress course, you'll be amazed what you learn, both what you can do, and what you thought you might, which is not so easy after all! Though I did not need to egress following the crash I was in, the confidence of that training, coupled with 25 years doing and teaching water and ice water rescue for the fire department gave me the best outcome in a very bad situation. It was my fire department colleagues who packaged me to the hospital, following the excellent initial rescue by the airport staff. Everyone said I was calm and answering their questions both in the water, and through out the whole event. All that training and practice had value for me!