I scaled up a recipe so that it would make a 10in and a 6in cake and I should have had plenty of mixture but it was was just over enough for the 10in.

So I started to puzzle over why it wasn't enough - couldn't think of anything so just put the one cake in the oven... 15mins after putting the 10in in the oven I realised I forgot the sugar! I didn't want the whole mix to be wasted so I whipped it out the oven and mixed in the sugar - but some of the cake had started to cook and go spongy already...

What do you think - am I alright? It is for my sisters 21st so its not for a "customer" no one is paying for the cake (well I am I guess lol) don't want it to be awful though.

I guess you'll have to start over again. I think I did that once, forgot an ingredient and tried to salvage one pan...and of course the cake fell. The cake would still be good... just a bit bland I guess. But the icing or frosting would make up for it. So if your family isn't very critical as mine, then you can still serve the cake. It's more like, you learn from your mistake. Good luck