I'll have to think about this...for now assume you're back as a normal robot person and we'll work out the shape shifting stuff later. I am leery of having more then one "The Thing" around, or potential the things. However, if you can think of some good limitations and downsides to it, then I'm a lot more likely to go with it. Powers must always have drawbacks. Because the universe is theatrical.
Limits I'll self-impose regardless:
Saint doesn't want anyone to know that he's unusual. He'll lie and say he got his body remade by the doctor. To uphold the illusion, he won't do anything obvious that risks discovery. So he won't regen wounds (unless he can cover it up using a medkit, and even then he'll leave some damage so he doesn't have to fake a limp or whatever), won't alter his appearance, and certainly won't grow organic scythes and start playing Prototype. He may do some internal remodeling, but nothing that would be visible from outside with a cursory inspection. He may stage his own death at some point, and impersonate someone else. Probably only if he gets discovered.
Of course, if he thinks he can do something without being detected, he might. For example, moving his brain to his torso. Or killing someone when completely isolated.
No external lifeforms, unless they're entirely controlled by electronics. Saint doesn't trust other brains, even if they're technically his.
Limits that wouldn't hurt me much because I probably wouldn't break anyways, but would force me to be more balanced:
Maybe say the Doctor decided to try creating a more 'stable' lifeform? So it takes a decent amount of time to make changes- I would be incapable of repairing large wounds fast enough to not die of bloodloss, but could still rearrange all my organs if I have an hour.
Say Saint's brain is incapable of controlling as much tissue as Xan is (Perhaps because of a damaged Amygdala?), so I can't increase my mass beyond 'normal human' size.
Things I'd dislike, but would certainly help balance:
Maybe Steve disliked Xan's ability to create standalone critters. So he told the Doctor not to make anything self-replicating. So any organic material which leaves my body automatically reverts to basic proteins. I wouldn't be able to restock my medikit with organic lookalikes, or launch effective projectiles. (I had an idea for assassination, if Saint could manage to get someone alone and far from cameras. He could turn his ribcage into what amounts to a directed frag grenade, and hug them. Or do it right before coitus, if he can seduce whoever.) It would also make remaining undetected harder- can't really explain why your arm melted after it got chopped off.
Have the Doctor force Saint to act as his agent for whatever he wants. Not sure how Saint would help him... Maybe keep track of his other creations? Try and convince other people to get experimented on by him? Semi-assimilate people he repairs on mission and commit experiments that the Doctor wants to try, but couldn't get volunteers for?
NOT giving me the robobody style brain texting thing that Xan got.
Make it impossible for my body to interact with electronics. This might already be in effect, but I was gonna try connecting Saint's nervous system to his wristpad.
Saint steps out of the infirmary in his shiny new robot body with a

emoticon. He experiments with the body for a short while, before changing it to a

emoticon.
I am a Fleshtech, specializing in Auxiliary systems. I currently have a robotic body and an emergency kit, although I have exactly zero real combat capability. I am willing to join a team, but I would require protection.
Charge an AUX bonus. Ask Steve if it would be a bad idea to join boarding without any weapons or armor.