Can somebody please explain the halacha of altering a tallis katan? If the garment is too wide but the length is good, is it OK to have it taken in? This is not a substitute for my rabbi's advice, just additional information.
I'm not sure what you are altering, so let's deal with a couple of options. I will assume the tzitzis are already tied on.
One may alter the material as long as it was a proper four cornered garment beforehand,and he keeps those same four corners as they are, see Shulchan Aruch siman O'ch 10 siff 5 and 6 with the Mishna Berurah. We must avoid a case of taaseh vilo min ha'asui, having string on the material and only afterwards turning it into a proper for cornered beged. That would be passul.
The part that goes over the shoulders can be more narrow than the rest of the material, but 'the part that goes over the shoulders should be wide enough to be apparent that they are part of the article of clothing and not look like straps'. Mishna Berurah siman 16 end of #2.