There is not any English translation of Yerushalmi that’s free. The only not free translations that are out there is Jacob Neusner’s which I am not sure will help you very much, and Artscroll. I don’t have either one so can’t send you the relevant pages. Sorry.

If you just need Mishna Kilayim 1:4 without the Yerushalmi, in English, you can download that here.

As far as your question goes about grafting of olives it goes as follows. Yerushalmi Kilayim 1:7, Vilna Daf 3b says:

תני אין מרכיבין זיתים ברכב של תמרה מפני שהוא אילן באילן. רבי יודן בעי ולית הדא פליגא על רבי לוי (תהילים קכח) אשתך כגפן פוריה בירכתי ביתך כשתילי זתים סביב לשולחנך. מה זיתים אין בהן הרכבה. אף בניך לא יהא בהן פסולת. הא מכלל שיש בהן פסולת. שנייא היא הכא שהוא עתיד למתקה כהדא רבי שמעון בר’ הוה משקי פרסתקיה יין מבושל בשביל למתקה

My translation:
We learned in Beraita (Tosefta Kilayim 1:7): We do not graft olives onto a graft (i.e. branch) of a fig because it is considered grafting a tree onto another tree. Rebbi Yudan asked: Does not this [Beraita] argue on Rebbi Levi [who said the following?], “[It says] (Tehillim 128) ‘Your wife is like a wild vine. I have blessed your house like olive seedlings surrounding your table.’ Just like [the prohibition] of grafting does not apply to olives, so too you will not have bad (literally: garbage) sons.” But we learn from this Beraita that there will be garbage (i.e. new “bad” fruit) from them (i.e. the olives)? The reason this Beraita was learned here was to teach you that [by grafting olives onto figs] he (i.e. the farmer) will sweeten them (i.e. olives) as [we learn] from this [case]: Rebbi Shimon Ben Rebbi used to water his peach tree with boiled wine in order to sweeten it.

Pnei Moshe explains this whole discussion as follows:
Rebbi Yudan thought that according to Rebbi Levi the prohibition of grafting does not apply to olives, because if you graft olives onto any other tree, it will not make new fruit. And if you graft another fruit tree onto olives it will continue making olives, so either way a new type of fruit was not created and therefore this is not considered grafting. This clearly contradicts the quoted Beraita according to which grafting olives with figs is forbidden. The Gemara answers that the reason the Beraita forbids grafting olives with figs is not because it will make a new type of fruit, but because even though the olive tree with the graft will continue to make olives, their flavor will change as a result of the graft and they will become sweeter. This small change is enough for this to be considered Kilayim and therefore is forbidden. However, all Rebbi Levi meant was that the olive tree does not produce new “bad” fruit as a result of the graft and therefore his metaphor with children still works. He agrees with the quoted Beraita though that there is a prohibition of Kilayim that applies to grafting olives with another tree. The Gemara finally proves that sweetening olives by grafting is possible from the case of Rebbi Shimon Ben Rebbi who used wine to water his peaches and as a result caused them to be sweeter, so we see that fruit on a tree can be made sweeter by adding to it stuff from another sweeter fruit (i.e. wine from grapes).

My comments:
It’s important to remember that the Gemara does not require scientific facts as proof for its assumptions. Merely stating that there is an oral tradition of the proposed claim is good enough proof, despite the fact that the oral tradition itself is flawed or inaccurate. The fact is that olives can be grafted with other olives to change the cultivar type (see here), but that would not be Kilayim since both cultivars are still olives. You can also graft wild olives with domesticated olives. I am not sure what the Yerushalmi would think of such a graft. The Romans were very aware that olives and figs can be grafted together. See here and here for quotes from Columella and Pliny. From just some quick perusal on the web it seems that a lot of people are saying that such grafts are impossible, because the trees are too different from each other. I am not sure what the latest research on the matter is and what the limitations are. I will leave it up to you to dig further.

Hope this helps.

Regards
Eli

Yerushalmi Yevamos with the commentary by R’ Shlomo Friedlander
Binyan Yerushalayim on Yerushalmi Agadeta by R’ Noach Chaim Levin
Kevod Chachamim on Yerushalmi Agadeta by R’ Shimon Wolf
Yafeh Mareh on Yerushalmi Agadeta by R’ Shmuel Yaffe – Venice edition
Yafeh Mareh on Yerushalmi Agadeta by R’ Shmuel Yaffe – Berlin edition
Yafeh Mareh on Yerushalmi Agadeta by R’ Shmuel Yaffe – Amsterdam edition

I need to update the seforim list, because it’s still missing a lot of books.

נספח לתלמוד ירושלמי כתב יד ליידן כולל שני מאמרים מאת פרופ’ שאול ליברמן ומפתח השוואה לדפי הדפוס מאת ישעיהו וינוגרד
שוב על כת”י ליידן של הירושלמי מאת פרופ’ שאול ליברמן, 5-15 [11 עמודים]
ירושלמי הוריות מאת פרופ’ שאול ליברמן, 16-27 [12 עמודים]
מפתח השוואה לפי הדפוס [קראטאשין תרכ"ו] מאת ישעיהו וינגרד, 28-47 [20 עמודים]
BM498 M3 L45 1972