I'd wait about a week. What's the filter in the 75? If you can, take a sponge from it and squeeze some dirty goodness into the water.
Or better yet, if the sponge from the sponge filter is removable, add it to the 75's filter if you can.
A few weeks is typical to be fully seeded. It's not a real quick process. Of course it'll have some bacteria sooner, and since it's only a fry tank, it might be ok to move it sooner. You could move it and test it by adding just enough ammonia to read .25ppm.