מותר חורבן הכרם - This is referring to an incomplete קרחת הכרם from פרק ו משנה א, which must be sixteen אמות.  If short of that figure, planting in its excess, that is in the middle space of the clearing that does not lie in the four אמה workspace of the surrounding vineyard, is prohibited but will not cause condemnation.  Similarly, the 'excesses' in the rest of this משנה are references to other cases given earlier where, if less than a certain size, were said to prohibit planting.

אין אדם מקדש דבר שאינו שלו - The רע"ב points out that this אסור is different than others like בשר בחלב, as here the פסוק specifies that the כלאים must be in his field, and not his colleague's - it says שדך as opposed to שדה.