It would help us to know what version of Finale your are using (and what OS and version thereof), as well as in illustration (scanned image?) of what you are looking for. I am guessing this is not simply a matter of accelerating the tempo.
2) yes the other instruments perform during this passage
3) it depends... but they play rhythm normaly (strictly in tempo) some of them quarters, other eights or nolets...
4) yes it is important that the 13th beamed note (= a♭) happens on the first beat of the second measure, right? And it is important that the 21st beamed note happens on the first beat of the third measure.
But what I want is only one group of notes because it is takes a very long time to extend beams from one bar to the next bar mostly if the beams are not parralel... You know what I mean ?
… it is important that the 13th beamed note (= a♭) happens on the first beat of the second measure. And it is important that the 21st beamed note happens on the first beat of the third measure …
1) As a simple layout solution, use only stems (= no beams).
Then, use the Smart Shape Tool to add false beams as Custom Lines with the same Line Thickness as beams.
To find out the thickness of beams, look in the Document Options.
2) The demands for the timing of the 13th note and the 21st note will make it physically impossible to perform a continued rallentando where the speed never goes up, always goes down.
You can perform a rallentando during the first measure so that the 13th note happens on the first beat of the second measure.
But then, to make the 21st note happen on the first beat of the third measure, you have to speed up again.
If you, from the beginning of the second measure, continue at the (slower) speed you have reached by the end of the first measure, then the 21st note will happen later than the first beat of the third measure. And the 21st note will happen even later if you also perform a rallentando during the second measure.
Since Finale can't do this beaming easily, I would put all of the notes from the three measures in the first measure all beamed together. Completely over stuff the measure. Then with Special tools, drag each note into your desired position. Then use Special tools to create the feathered beaming. It's clumsy, but it gives you control over the beam.