I tried using the mantra So Ham with the small universe meditation- I find the flow from point to point easier to follow. I am curious if this is recommended or whether only to use the Omm and Mauh and use them only on the inhale
That should be alright. If you experience negative or strange effects, just simply don't do it anymore. If you find it helpful, do so, as long as the technique is simple and meaningful, go for it.