I'm not familiar with Ugro rhizo. But I looked it up and it looks like it is coco coir. It comes in a brick form that you saturate to loosen, then let dry. I used to purchase coco coir in this form from a pet supply store as a substrate for my tortoise.

For aroids, most people will recommend a very loose, well-draining, airy soil mix. You could make your own with a mix of: coco coir, small orchid bark, rinsed perlite (preferably larger size) and a dash of your worm castings. Some horticultural charcoal is a nice addition as well.

There are a bunch of videos on youtube with substrate mixes for aroids. Or you could Google-up a bunch to compare. I found this one:

https://thebotanicalarchive.com/blogs/news/potting-mix-for-aroids

Would love to see a picture of your seedlings once you have them all potted up in their homes! Billietaie is of interest to me.

im not sure i understand how small or large these seedlings are ... it might matter ....

im not sure i would mess with the worm castings at the babe stage.. you might be further ahead using a very dilute water soluble fert with each watering

the problem with the worm casting.. is we really dont know what it is .. other than it came out of a worm .... its not predictable ...

when ever you are doing something .. an experiment lets say ... you try to know and control everything... so you can predict the end result ...

this is why we usually avoid using garden soil in pots .. because how it retains or sheds water in the pot ... so we cant predict how to deal with it ... and often.. by the time you figure out how it works.. the plant is already struggling ...

i might GUESS that if you dont want to go to the expense of coir.. a general cactus mix might work ... as i read the situation ... high drainage.. seems to be the key ...

ken