I definitely agree with this. They were may e trying to save some lower to midlevel summons for later games while having some kind of ultimate summon for this game, hence bahamut, but I think it would have been better to treat Ramuh as something of an ultimate summon for the first part. They may have also been thinking of working with the fact that FF7 has three bahamuts and using one bahamut per part (if they do three parts) but that doesn't justify having leviathan too.