I don't really understand that statement "... I never use the horns from Brass 1 because they are different..." Maybe I am not clear on which particular instruments are being spoken about in this post. But anyway the 4 horn ensemble is perfect for splitting the 8-horn ensemble into two 4-horn sections. In fact, that is the go-to setup for horns in large VSL orchestra, because of
1) the 8 horn ensemble sounds so good you always want to use it for general unison orchestral purposes, and
2) more often than not, divisi in horns is best done in two part rather than four, because of balancing with the other brass in a tutti. 4 part divisi is best used with very soft or soli passages because a single horn playing will be overbalanced by other single brass if you use all four players on different notes in anything louder. I derived this practice from my own real horn playing in orchestras - if you write all different notes for each horn against the other brass in an ff section, it will not be balanced unless two players play each note in unison in an 8 horn ensemble which of coruse is the reason that Mahler started using 8 horns.
So for these reasons, the ideal combination of instruments is to have the 8 horn ensemble for a unison line and split it into the two 4 horn ensembles, then for smaller divisi the Dimension horns or solo horns could be used.