Interesting concept! And the puzzles, though small, are still difficult enough to be interesting.
I need a clarification, though. When the operator is addition or multiplication, the meaning is obvious. When the operator is subtraction, and there are only two digits, apparently either digit (not just the leftmost or uppermost) can be the larger.
But when the operator is subtraction, and there are four digits, does it mean A - B - C - D (which is the same thing as A-(B+C+D)), or are you also allowed to use parentheses, as in A - B - (C - D) ? If the latter is allowed, then your first puzzle has two solutions, because the 4-digit vertical word on the left could also be 6,7,2,1 since 6 - 7 - (1 - 2) comes out zero, as demanded by the clue.