1. Ilford filters are supposedly speed matched, except for 4/5 which require a doubling of exposure. They are close but not quite perfect. If you need to bump up a grade, make a test strip with those guidelines.
2. Buy your own set of filters. Any liquid will ruin them, even small drops of water or moist fingers.
3. The only disadvantage would be the smaller magnification for the 35mm frames. For example, if you print on 8x10, you'll find that you are having to move the enlarger head up and refocusing when switching formats, which will take more time than switching lenses.