@MetaGable in japanese folklore, dragons can transform into clams, and in this manga's lore, nightmare clams are technically still dragons. So when Thistle summoned every single dragon, it included nightmares!
The problem with two different species reproducing is that:
1.) Sperm can't fertilize egg
2.) Egg could get fertilized but offspring won't survive to adulthood
3.) Offspring that did survive to adulthood is infertile
4.) Offspring could be fertile but offspring's offsprings may not be and they probably carry sucky genes. Aside from being possibly infertile and having sucky genes, it just gets worse after every generation.