Single strand fiber transceivers/SFPs are also known as BiDirectional or WDM SFPs.  The primary difference between BiDi transceivers and traditional two-fiber fiber optic transceivers is that BiDi transceivers are fitted with Wavelength Division Multiplexing (WDM) couplers, also known as diplexers, which combine and separate data transmitted over a single fiber based on  the wavelengths of the light. For this reason, BiDi transceivers are also referred to as WDM transceivers.  Single strand fiber SFPs are installed in pairs, where the transmit wavelength of one end is the receive of the opposite side and the receive wavelength of one end is the transmit of the opposite side.
You can use 1.25Gbps single strand fiber SFPs on the new base and the splitter, making sure the pair of SFPs will talk to each other. For instance, one end will beTX: 1550nm; RX 1310nm and the other end will be TX: 1310nm; RX: 1550nm