Q1: absolutely nothing, you would simply loose some brightness that you can very well compensate with a longer exposure. I suggest keeping the 6x7 condenser and relative accessories, as I foresee that 6x4,5 will be not enough for you in the medium/long term, but on the other hand a 6x7 gear can be easily used to print a smaller format. The opposite is, quite obviously, not true.Originally Posted by kdanks
Q2: yes - 50mm for 35mm, 80mm for mid format.
Q3: the metal masks just support the negative and crop precisely the frame you want to print. Please note that you can print fine a smaller format with the 6x7 mask you already have. A smaller mask like 6x4,5 or 6x6 - though convenient - is not strictly necessary. You just need to take care in centering well the frame you are enlarging, and if you have a negative carrier adjustable in width you can crop the area outside the frame being printed (but this is, again, not strictly necessary).