Now I'm certainly not one of the experts, but I know the answers to this!... Just wait a while and the experts will come along though...
The Cougar is modular, ie a control board and a powerboard... So yes you can spec the powerboard to handle lower voltage, altough I'd probably keep that as a margin for upgrades and just lower the spec a little (say 96V?)... Just enough to save space...
The size of the Cougar is really up to you... The tighter you manage to buch things together the smaller it gets... Using the prefabricated powerboard it will be larger than the Altrax, but if you are swapping those components you could potentially slim it down a lot...
You connect to it using a com-port and software, I'm not sure if that software is windows only, but if so wine should solve that... The largest problem would probably be to find a computer with a laptop today, or a USB com-port adapter that works (they have a tendancy not to follow standards)...