[H500]

I bought a 24v 15A but while it doesn't arrive I'm using the 12v rail from an ATX PSU to test the steppers. Also, since I cannot use the steppers in bipolar parallel mode because it would draw 4.2A each I'm using them in bipolar series mode.

Bipolar parallel will not draw 4.2A. It would draw whatever your drive is set to.

Bipolar series with a 24v supply is a capital crime. If your motor inductance is greater than 1 mH, don't use it unless you enjoy watching your machine stall at low speeds.