If you are willing to do a little modification, you can trim many of the refills to fit with the other spring provided. The goal is to make the following refills the same length as the Schmidt refills, so lining the included Schmidt Easy Flow 9000 refill up with anything below and trimming them to the same length with a sharp Exacto knife (or something similar) will allow these to fit in the Slider:
Pentel Energel
Pilot Acroball
Pilot Frixion Ball pen BLS-FR5 (LFBK-23EF-B refill)
Pilot G2 (Americas #1 selling ink gel pen, 0.38mm, 0.5mm, 0.7mm, & 1.0mm)
Pilot G2 Pro
Pilot G6
Pilot G-knock BeGreen
Pilot Hi-Tec-C "Cavalier" Refill (PILOT LHRF-15C3)
Pilot Juice
Pilot Precise V5 RT/V7 RT, named Hi-tecpoint V5 RT/V7 RT in Europe
Pilot Precise Gel BeGreen Must fill in back space
Uniball Signo RT Gel (UMN-138)
UniBall Signo RT1 Gel (UMN-155)
Uniball Signo 207 Gel Refill
Uniball Signo 307 Gel Refill
Uniball Jetstream 0.7mm (SXR-7) & 1mm (SXR-C1)