First, Parsha: See 18:9: "Vayichad Yisro on all of the good that G-d did to Yisrael..." What does vayichad mean? Rashi says the pshat is he was happy. Rashi then quotes a midrash (really a Gemara Sanhedrin 94a -Perek Chelek) "that his flesh had goosebumps (vayichad) he was troubled on the destruction of Egypt. And this is why people say a ger - until 10 generations do not put down an arami (generic for non-Jew) in his presence."
I was thinking, what if this Gemara is based on our Gemara in Kiddushin 75a that "Rav Yosef says until 10 generations, a ger may marry a mamzeres. And some say until the shem ger is removed." This statement Rashi brings seems to be like Rav Yosef's statement. And then, (because someone asked me to look something up in the Mizrachi) I saw the Gur Aryeh (Maharal) who says this explicitly in Rashi.
I recently posted about the Machlokes Rambam and Ran about Ger marrying a bas yisrael, will the daughter be allowed to marry a kohen. And we've discussed that a ger can marry a mamzeres (and vice verca) - Be'er heitev 4:27 points out it's not prohibited (midioraissa) from the lav not to marry a mamzer, but he says it is prohibited (sounds like midirabanan) because we do not want to have more mamzeirim (i.e., they are allowed to marry, but their children will be mamzeirim).
I've been thinking - I think the Ran paskens like the Mishna on 66b [almost] all the way, but the Rambam does not pasken like that Mishna in all its details, because he pasken like later mishnayos on Daf 75 & 77 regarding ger & mamzeres & certain people who can marry a kohen (see E"H 4:5 & 4:19, I hope to discuss that machlokes rishonim in another post.)
And about 10 generations about Ger - those who pasken like Rav Yosef will say the male son of a male ger 10 generations later will be prohibited to marry a mamzeres (this is where all of the men married a giores or regular bas yisrael, not someone pasul). The Rambam paskens like the second opinion, not Rav Yosef, and he says such a person will be allowed to marry a mamzeres, because we don't follow the 10 generation thing. However, if any of his forefathers married a regular bas yisrael, the shem ger is removed (it could even be after 1 generation) and none of the descendants from that point forward may marry a mamzeres.