Actually, this seems like a potential problem. Even though they're both 4U format, Buchla and Serge are NOT ALIKE. They have different methods of powering, the Serge uses a single "plane" of control and audio signals while the Buchlas have their audio and control paths separated and with rather different levels (save for the Eurorack versions, according to Tiptop), and they don't exactly have the same "space unit" size. The issue is sort of similar to the already-resolved 5U systems, where the MOTM format and the "MU" format have different widths even though they're similar, or how Eurorack and FracRack can talk to each other (as a rule) while they're not the same height and same power requirement, ergo no Frac modules in a Euro cab...although older Paia 2700 and 4700 series modules ALSO use the same split plane as Buchlas.
This is actually a problem that's about as old as modular synthesizers themselves. Over the years, there's been a LOT of different module formats...E-Mu had one, Wavemakers had one, then there's the ARP 2500 which requires the matrix switches along with the modules, and that list goes on and on and ON. Plus, loads of these synths would NOT play nice with each other. For example, you have EML's patchable systems, they take 1/4" patchcords, everything seems right...until you plug in a pitch CV and discover that wonderful EML CV standard of 1.2V/8va. If you don't like microtonality, don't DO THAT.