懇請863211老師和皮匠老師和同學幫忙：I-485Pending後換工作

863211老師和皮匠老師，我在2006年初由公司A(a big company)遞交了Perm並獲得批準，隨後I-140在2007獲得批準, 在2007年7月遞交I-485。2009年初裁員, 2009年5月到公司B(a mid-size company)工作，於同月收到USCIS的RFE (Employment Letter)並由公司B回複(no response from USCIS)。後在2009年9月換到公司C(a small company)。現在又有一機會到公司D (a start-up company)。我的問題是：
(1)Is this a problem to change company so often (change jobs less than half year again)? (2) My current salary is lower than that in A (however, higher than the prevailing wage when I applied Perm), is it fine? (3) How USCIS judge my field is similar to that in Perm or not, just from the employment letter? (4)I can ask my manager write a letter, which can match the content in Perm but is different from daily job, how USCIS know that? (5) I often heard the status of “Pre-Adjusted” or “Pre-approved”I-485, what does it mean and how can I know whether my 485 is pre-approved or not? (6) If my 485 is pre-approved, will USCIS ask RFE again? (7) The current company is small (C or D just has a few employees). Is the size of an company an issue?

Thank you for your answer.