When my order for Rapier Miniatures Parthians was delivered, Rapier kindly sent me some samples. One was a pack of early Imperial Romans and the other was some American Civil War figures. The American Civil War figures were good as they kind of allowed a like for like comparison between the Rapier figures and Baccus/Adler Miniatures.
The first comparison is between the Heroics and Ros Prussian musketeer and some Rapier ACW infantry. A you can see there is a considerable size difference between the Rapier and the Heroics. I certainly would not consider mixing these figures together in the same unit.
Even thought the difference in height is about 1mm, it is very apparent which is the larger figure and which is the smaller.
When comparing against the Adler Miniatures the size difference is a little harder to see. Unfortunately the only figure I had easily to hand was a plumed figure. Certainly the Adler head is much larger than the Rapier head and this makes the Adler figure look larger than the Rapier overall. However, allowing for the height difference bought on by the large plume, there is not too much of a difference between the figures. I think that it would be possible to mix Rapier figures and Adler figures within the same unit.
Given that the Baccus figures look slightly less bulky than the Adler, I also think it would be possible to mix Baccus and Rapier within the same unit. I will be in a better position soon to make that comparison however as I have a batch of Baccus Sassanians on order and due for delivery in the next two or three days.