Think of index shifting as a system. The derailleurs are designed with a certain pull ratio, which the the shifter drum matches while also providing ratchet stops compatible to the number of gears to be shifted. The jockey wheel on the RD may also be designed with a bit of 'wiggle' to ease gear change and gear teeth designed to engage and release the chain quickly.

You maybe able to go indexed with a set of shifters and a matching RD.

Brad