If creatures compress down to 1kb then I guess the game limit could be enormous, it could be limited just by disk space available to the end user - you could wind up with millions of the things!

I don't know if how many creatures a person would make is an answerable question without having the game, that depends a lot on how long it takes to make a creature and that's totally open ended. For sure its one of those things that someone could run up something good in a very short space of time, equally you could probably spend hours trying to perfect a complex creature or get a specific look to something. The editor looks pretty simple to just make any sort of functional creature, you could make something really funky by accident for sure, but I bet if you wanted to make like say Godzilla, it could take quite a long time to get it to look just so.