OK... Looks like the task is not common enought.
After doing some more or less random attempts I ended with:
* project set to 100 BPM
* items forced to be 133 BPM and 6/8 time signature.
But I failed to make items "not forced" to anything, matchting project settings instead.
Finally I re-recorded midi items, using original items as source, recording to new tracks. Now I have a 6/8 song played with 100BPM. In fact, it should be 200BPM (see below), but I needed to divide measure by 2, to support dependency between BPM and a VST.
BTW how does it work, that item can be fixed to some tempo/time signature while its playback is controlled and affected by project settings? Is there a description of relationship between those settings? What happens if recalculation of tempo results in some divider reminder?
Anyway... trying to achieve the goal I hit a wall of lack in education probably. What surprised me most is that BPM of 6/8 and 4/4 songs are different even if time of playing a single measure (bar) seems to be the same.
Let's take Oxygene pt. 4 for example. This is 6/8 song with 189 BPM. But if you measure time of single measures (bars), it's like 126 BPM.
I even saw a discussion on gearslutz about it but seemed there was no common agreement
Could anybody help me to understand this subject?