Your starting hand will include 1 Lvl 7 or higher DARK spellcaster-type monster, randomly selected from your deck. This skill will not activate if your DECK does not include 1 Lvl 7 DARK spellcaster-type monsters. You cannot special summon effect monsters during your First Turn.
REPEAT: YOU CANNOT SPECIAL SUMMON EFFECT MONSTERS DURING YOUR FIRST TURN.
So, you did use Red-Eyes Fusion and then "for some reason" you cannot summon any other cards anymore? That's crazy. I wonder if Red-Eyes Fusion has some "hidden" mechanic going on.
1) You summon Cosmo by its own effect by sending a monster to the GY.
2) Cosmo's 2nd effect activates. It checks the monster's levels that is now in the GY. In this step, you don't care anymore what level it had in step 1) because this step is a different effect.
FYI: That's also the reason why you can use Embodiment of Apophis for 1) but you don't gain ATK because in 2) it is a trap and not a monster. That is the official ruling of this card:
https://yugipedia.com/wiki/Card_Rulings:Cosmo_Brain