Hi @CobaltWolf,
I have a suggestion for 3-4x additional parts which would I think would allow both the Big-G variants above to be recreated in KSP to a pretty good approximation.
https://drive.google.com/open?id=1E5m6fRmHbDXWWLTO1_0whF7HUCioLVLP
The top variant of Big-G (to fit onto an SIVB upper stage) can be easily created I think with 1 extra part. Something like a Medici Service Module, with the profile of the SIVB interstage scaled down to a 3.75m opening diameter.
https://drive.google.com/open?id=1DSsZWtD6on--PfDrNzaP631VaWHbDi9l
The lower / Space Taxi variant - I think my model is slightly too long but I would suggest a service module, similar to the Kane CSM, with a 2 kerbal crew capacity module in the rear for realistic docking operations. This could be a single part, or two parts for the Service Module and Crew Module elements respectively - for maximum reuse potential.
I don't know why but I've always thought the MOL docking ring most appropriate for the Space Taxi, although I don't have any evidence to support this.
https://drive.google.com/open?id=1LwmxxIX9RoLcXEa_aaCyOsDzqcHOXscq
The final module I think could then be something similar to this Orbital Manoeuvring system (sorry I can't remember where its from - might be stock?), but scaled to the profile of the Inon adapter tank in the previous model. A monoprop engine cluster with monoprop CSM and docking module would then make a pretty good looking space taxi! :-)
Edit - I think there's enough variety in the Kane CSM module variants that you could get away with adding just a half-width orbital module without the tapered faces as a "CSM Crew Compartment Module" or something like that.
@CobaltWolf the more I think about it a half-size Botticelli Crew Module (with 2 Kerbal capacity) and accompanying heat shield, to convert the Leo to 1.875m, would then allow existing parts to recreate the Rescue Gemini variant. Perfect for completing Kerbal rescue contracts or retrieving that Leo Minmus Flyby crew that couldn't quite make it back to Kerbin!