Repeating small word like ON is usually not a problem. Even if they intersect (which is undesirable), the puzzle may still be accepted if the editor likes the rest of the puzzle.
However, the 15-letter phrase ON A SCALE OF I to IO, in my opinion, will likely be a deal-breaker for this puzzle. It would be fine for a themed puzzle that uses I's and O's as numbers and letters in the theme entries, but in a regular themeless it's pretty much a no-no. This is because the I's and the O function as numbers in one direction (and I'm assuming) letters in the other.
Roman numerals don't have this problem, since they are always letters. Therefore they can be a number in an Across and a letter in a Down.