Question is it legal to play the rebel comms specialist as a mini in a rebel trooper squad even without the upgrade card on it's unit , the reason is just that I hate the rebel troopers with a vengeance an if I can replace them with a better sculpt I would.

I'm interested in this as well. With the new heavy upgrade troopers coming out for all factions it would make sense that they could play as a basic squad. It even comes with a trooper unit card.

There are no detailed rules for this. So the general answer is always: check with your opponent / ask the tournament organiser.

As long as it's unmistakably clear what a mini is used as, there should be no issue and for friendlies it should be no problem anyhow. But if you want to take it to a tournament, especially a high level tournament, any possible figure confusion can lead to a bad experience and should be avoided.