I think almost any paint or dye could be used, I think textile paints such as Jacquard would need to be diluted a bit. I used Dye Na Flow and did dilute it by about 50% but also used it full strength and it came through the felt tips just fine.
Filling the markers was a bit of a challenge at first, I didn't realize that not only the top needed to be removed but also the valve that controlled the flow. Once I had done that I used a cup to hold the marker still and carefully poured the paint into the reservoir. There are two glass ball bearings inside to keep the paint mixed.
I began by pinning my fabric to the paint table but found that it moved too much when using the marker, I needed to push down on the felt to open the valve and start the flow of paint. I solved this by ironing the fabric to freezer paper and pinning it all to my print table (the yellow blob on the centre-left was when I forgot to replace the value that controlled the flow!)