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?
Although switching the old tzitzit for new is a good thing as we find in Yabia Omer 3 Yoreah Deah 18:5, שו”ת חינא דחיי סימן יט, and the Levush who all write that it is allowed and even praise worthy to switch the old ones to new ones. This question is also related to having different lengths of strings in the tzitzit and I'll explain why. In both the Magen Avraham Orach Chaim Siman 11:14 and the Mishneh Beruah Siman 11 Seif Katan 68 they quote the Rosh who says, שיהיו כל החוליות בשוה שזהו נוי ציצית, that all the strongs should be equal in length because that's the beauty of the tzitzit. We see that having the length of the strings should be equal, to show the beauty of the tzitzit which is similar to the concept of זה א-לי ואנוהו. There should be no difference with regard to the thickness because it would be the same halacha, and we would apply the above idea. Therefore it seems to be okay to wear such a begged, but you should be aware of this concept of זה אלי ואנוהו. I’m not telling you what you should do, but making you aware of the details of your case, so please ask your LOR.
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.