Could I gain weight/fat by what I just ate?

OMG. If your really that worried then go on a walk for an hour or two. Its not like you eat food and suddernly you gain two llbs. Why don't you go do some chores, that burns calories.

By the laws of how things work in this universe:

If you put something inside of you, you will gain weight. The weight gained will be however much all the rice cakes weighed.

If those rice cakes contained fat, then you will gain fat. Otherwise, where else would all the fat go?

if you do this alot then yes. if it was just this one morning, i believe it should not really affect you. if you do some mild excercise today it will totally offset the bad effects

you won't gain any weight if you won't consume more calories than you burn.