Hello snow-runner yes you can grow spelt and oats for juice.There nutrient profile is very good but there germination rate for sprouting is notoriously poor.Spelt and oats dont handle the processing well and is the main reason they have fallen out of favor from a farming perspective.If your buying direct from the grower you are truly blessed and should try some for sprouting.Pea sprouts are sweet and are used as an alternative for wheatgrass juice at the hippocrates institute in florida.The best seeds for growing grass are wheat,rye,tritacale and kat.
I compost my flats outside in an open air compost pile.Very simple and effective but the turnover time varies on weather conditions.The heavily bound root masses can be physically broken up to enhance the composting process.They have composting drums and other contraptions to make composting more user friendly.
I use kelp in my dry mix along with azomite a powdered rock.Then I add 4 cups of vermiculite and 12 cups of recycled compost that was run through a wire mesh to break up the clumps of root mass.Recycling flats is labor intensive but well worth the effort.