Alternate instrument (small) notes

Put the BCl/Bsn notes in voice 2 (see https://musescore.org/en/handbook/3/voices). Then untick voice 1 in the selection filter and select the range of the new addition. The inspector will give you the options of working with notes, grace notes or rests. Select notes and tick the "Small" box in the chord section (don't use the notes section as that only applies to the note heads, not the stems or beams etc.) Then repeat that and do the same for the rests. When done, re-tick voice 1 in the selection filter or you will find it impossible to edit the barisax notes.