How old is the terp?...it doesn't need brackish water. but so you know, the common musk has a very low salt tolerance and will not do well if you add any salt.
terps all adapt fine to fresh water unless they are wild caught adults and even then it can be done if you are careful to watch for shell problems and the blaoting associat6ed with osmoregulation. of course a hatchling will be easier to get started in fresh than an older terp.
Also any terp (even a male) will out grow a 55 in a few years.
They are both highly carnivorous so that shouldn't be a problem.
BUT the terp is diurnal (though i have seen them in the harbors on Long Isalnd at night) and the musk may tend toward crepuscular or nocturnal habits. Te depth thing might be an issue as the musk will probably like a shallow area in the tank wheras terps have a tendancy to get fat in captivity and they need as much exercise as you can give them.
i think multiple basking spots ans a partially submerged platform (of plastic or cork bark flat for example) might help for the musk not to be shy of the terp and to have an area of shallows to walk about in.
I think its funny how its everyone else's fault if something goes wrong because we didn't warn you. ; )
I think it will be fine though.