I have been wondering for some time about this, Is it really possible to use any shampoo solution (i.e Bissell, Hoover, Rug Doctor) on a, or rather, any Kirby G series vac?
No. Kirby vacuums can only use a dry foam shampoo. They're not easy to find, outside of the Kirby brand. I've found some before, Franklin Super Carpet and Upholstery Shampoo. It works well, smells somewhat of bananas, but not unpleasantly.
Most importantly, the other products you've mentioned are intended for wet extraction, which a Kirby is incapable of. Dry foam gets into the carpet, captures and encapsulates the dirt, and then is vacuumed up when dry. Use a wet extraction product, and you'll just have a mess that won't come out of the carpet.
If you don't use dry foaming shampoo in your Kirby, you'll end up leaving a lot of soap residue in the carpet. All that soap residue will will just attract more dirt and grim. On the other hand, dry foam shampoo is scrubbed into the carpet but leaves the carpet fairly dry. When it is completely dry, the carpet is then vacuumed to remove the soap residue and any dirt that it gets out. That won't happen if you use extraction shampoo.
it could damage my kirby?!?!?!?!?!?!?!?!?!?!?!?!?!?!?!?!?!?!?!? if there is damage to my kirby, does that mean that starks scamed me into breaking my kirby so they can make more money fixing it????? i have been using the wet foamy stuff for 4 to 5 years now