Fixed step pushing hands means you don't take steps. You have to learn to yield on the spot.
I've come across a lot of people who gloss over single push hands in their hurry to do double push hands and push hands with steps.
They all have their place and value. Single push hands explores quite clearly the energies of ward off, roll back, push and press ( or squeeze). This needs to be practised with lots of different partners to get a feel for how different people are in terms of energy and physicality.
If you don't investigate and understand the energies of single push hands, what chance do you stand in the rush to do double push hands etc.
Whilst single push hands looks simple, it is actually very advanced.
Play with that!