The idea of getting small units to fully utilize the available space sounds good but you end up with a very cramped work surface so I have moved away from that.
The first unit I got was a Behringer 130 Dual VCA, you can never have enough VCAs. To complete the top row I also went for a Behringer 121 Dual VCF. To make room I relegated the original Behringer 110 to the second rack and thanked it for its service 😉
I also added a couple of Erica Synths Buffered Multiples, these have a very useful voltage shift switch that will act to shift pitch on Oscillators.