I have a question. As I've been leveling and looking forward to getting to the more iconic ships from the shows/movies. I was wondering is it a '"good" idea to use say a exploration cruiser at rear admiral instead of the star cruiser? I understand you have your choice between assault and star cruisers at rear admiral.
There was a video from STOked where Zinc said that you could outfit a Miranda (starter ship) with Mk X parts and be an admiral, and still play through the whole game. It would just get harder and harder as time went on, so yes you can stay in lower ships if you wish. However, as Sorphius said, you lose out on weapons slots, bridge officer slots and console slots, and of course the starter ships aren't as good as the T5 ships by nature.
I kitted out my tactical escort in Mk X equipment and did some borg deep space encounters, it worked ok to a degree but i was very squishy. There really isnt much point to playing with a lower tier to be honest when tier 5 has access to more skills I was just a bit bored.
I guess it was a bit fo a challenge soloing a borg spere was a bit close.