Oooh right.
Well, the synth/with_synth/use_synth/play commands all expect to play synths created with the language that Sonic Pi’s sound engine, Supercollider, provides. So the solution there is either to search the Internet for so-called Supercollider ‘SynthDefs’, or build your own. (Which I am actually in the process of doing at the moment!).
However, the alternative to this is to use Sonic Pi’s communication abilities to talk to external programs that can run other virtual synthesiser plugins.
Referring to the tutorial, sections 11 and 12 (or Sonic Pi - Tutorial and Sonic Pi - Tutorial), you’ll see that Sonic Pi has built in support for the MIDI and OSC communication protocols. Any virtual synth that accepts either of these protocols will work.
There’s probably a bunch of information floating around on the forum here about ways people have done that - search for VST, MIDI, or OSC etc. One such example which may give you an idea:
Is it possible to control via Sonic Pi an instrument in Ableton?