Plants help a lot. Specifically fine plants near the surface. Hornwort is a great option. Be VERY mindful of floating plants covering the surface. If the water gets hot, and the surface is overrun with floating plants you have very little gas exchange happening in water that already doesn’t way to hold oxygen. I learned this the hard way this summer with medaka (also my first time breeding outside) and I got up to around 250 fry. Then my dad had an emergency with his 600 gallon reef that required me to be away for 4 days in the height of Tennessee summer heat and all but one fairly mature fry died. I only lost one adult because medaka are so tough, but the fry aren’t so resilient.
I think my biggest issues here were not staying on top of the the floaters, tub placement (got full sun in the middle of the day), and my tub being relatively small. I was working with a 27 gallon tub and that volume of water got hot quick. I found though I’d get a very distinct thermocline about 5-6 inches below the surface where the water was much much cooler and didn’t feel like bath water. The adults were fine down there but I don’t know if the same would work for fry. I’ve now got them back inside and finally just last week got fry again. Not many fry right now but I’ll get there.
If you’re trying to colony spawn and have the babies with the parents heavily planted in the way, but if you want them to lay on mops I’d use riparian plants in pots like aquapros does on YouTube. I’ve found they very preferentially lay on plants over mops if they have the option.
All in all I think you’ll probably have success. Medaka are easy to breed and the adults are the toughest fish I’ve ever worked with. I’d definitely start with the 16 fish as you’ll expand your population faster, but also have broader genetic diversity. Good luck!