+1 to what rich said
when i reared gemmatus, i had 2 distinct bloodlines at 2 different times, 1 from d17oug18, and 1 from yen, yen's bloodline was the second bloodline i kept
with each bloodline, i got presub or subadult nymphs, they matured, mated, and i had L1 nymphs once for each bloodline, because when they matured they failed to reproduce, but this is irrelevant
I used mels with the L1 nymphs the second time around, here's why
the first time around, the ooth took a long time to hatch, so my melanogaster culture died, and i only had hydei while some of the nymphs were strong enough to handle hydei, others couldnt hold on to the larger type of fruitfly
and since i only had hydei and it was too cold to catch mosquitoes, only those that ate hydei ate at all and survived to L2, where they all handled hydei without issue
the second time around i used both, that way the ones who couldnt eat hydei though there was nothing wrong with them, could eat mels and survive and i would have more nymphs/adults in the end
at L2 they all were eating hydei and by L4 they were eating bottleflies as big as themselves though it took a while to kill one and i once saw 2 L4 males eating the same greenbottle
so it is kind of up to you, they can eat hydei, though a few may have trouble and you may have a slightly higher L1 mortality rate like i did by using hydei alone at L1
however, i notice that creobroter are prolific breeders and any L1 nymphs lost will certainly be made up for when the females reach adulthood and produce nymphs of their own