I think they were probably equals and Robert's scruples fancying that she might be a little above him was because he was "a man in love".
Mr Knightley even said --
"My only scruple in advising the match was on his account, as being beneath his deserts, and a bad connexion for him. I felt, that as to fortune, in all probability he might do much better; and that as to a rational companion or useful helpmate, he could not do worse. But I could not reason so to a man in love, and was willing to trust to there being no harm in her, to her having that sort of disposition, which, in good hands, like his, might be easily led aright and turn out very well."
Mr Knightley also said something which rather puzzles me about the writing master --
"Let her marry Robert Martin, and she is safe, respectable, and happy for ever; but if you encourage her to expect to marry greatly, and teach her to be satisfied with nothing less than a man of consequence and large fortune, she may be a parlour-boarder at Mrs. Goddard's all the rest of her life -- or, at least, (for Harriet Smith is a girl who will marry somebody or other,) till she grow desperate, and is glad to catch at the old writing master's son."
I wonder what exactly was a writng master and his rank in society.