Q. 1) Why is it that at many kidushin they usually serve hot fleishig cholent on the same plate people ate their shmalzherring, is that correct?

A. Shulchan Aruch (Y. D. 116: 2) based on Talmud (Pesachim 76b) rules that one must be careful not to eat fish and meat together because it may cause tzora’as (dovor acher, as Rashi explains). Although, it would seem from the Remah (ibid.) that the abstention is only when they were cooked together, many Poskim assert that one should not eat them at the same time even when not cooked together and so is the accepted tradition today.
Magen Avraham (Orach Chaim 173) writes that there are many natural phenomena mentioned in Talmud that no longer apply today. It would seem that so is the opinion of the Rambam, who omited this concern altogether. The Chasam Sofer (Tsh. 101. and Shulchan Aruch Y.D. 116 ) offers two explanations for the omission of the Rambam. First, he suggests, it is possible that the Rambam knew that the Talmud was only concerned with the specific fish (Beinisa, known to them), but all other fish really pose no danger when mixed with meat. Alternatively, Chasam Sofer suggests, the Rambam knew that nature has changed and although there ones was a legitimate health threat posed by mixing fish and meat, no such threat exists today. Kaf Hachaim (173: 9) maintains, that it is true that the nature has changed, however, it may have changed for the worse and may still be a peril. He adds that “One should be more stringent in doubtful health issues, than on kashrus matters.
Horav Shlomo Miller’s Shlit”a opinion is that definitely two plates should be provided, as correct caterers do; to discard one after consuming herring or fish. In case there is no clean plate available, if only the oil remnants (no fish parts) remain on the plate, one may clean them with a napkin, and eat the fleishig cholent, on the clean corner of the plate if possible.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a