You only need to hit the approprite chord type slighty before or even once the fill has started, (or anytime during the fill for that matter). The actual content of the fill is not necessarily minor, 7th or anything else; it's whatever you choose to program in there.
The original intention, since the names of these fill riff are MINOR, 7th or whatever, was for you to program a fill which features appropriate notes for minor or 7th tonality. However, since we're using this feature for a different purpose you can program whatever you like into these riffs and that's what you'll hear when you hit the fill.
Basically each fill riff is an empty eight track sequence. you can program whatever you want in there.
If the minor and 7th fill riffs are empty, (as is the case with most factory styles), then, if you hit the fill button while playing a minor or 7th chord, the machine will simply take the one fill riff which does exist, (usually the major), and temporarily modify it's content to make it sound minor, (by flattening thirds), or 7th, (by adding flattened 7ths). However, if the minor or 7th fill riff does contain something then you will hear this exactly as programmed.
Hope this helps
Chris
P.S. For many factory styles, you may actually find that the MAJOR FILL riff is empty and the one and only fill for each variation resides in either the MINOR or 7th FILL riff location. Some of our programmers found that this facilited better conversion depending on the content of the fill.