If I am replacing the tzizit on a begged and I use different thincknesses of strings for different corners, is it okay to wear such a begged? Specifically, if 2 corners have thin, dak strings and 2 have thick, aveh strings, is there any problem wearing it because of the inconsistency in the appearance?
Seems like it should be fine. Meikkar Hadin (letter of law), I believe the Mishna Berurah writes in Hilchot Tzitzit that you can even have different Shitot (ways of tying) on the different corners. All the more so just to have different sizes! However, different methods of tying is not recommended per the Poskim.