Lets say you enter a pet in the spotlight and it wins, can you alter it later on without loosing the trophy?

For example: Lets say I enter my Ginger in the spotlight and win, can I later on go back and say change her minion or add/remove stuff from her TC without loosing her award?

, yes. Once you win the trophy you keep it no matter what you change.

Thanks just wanted to be sure because people keep telling me I should enter my pet but his treasure chest is small and expensive for certain stuff I might not be able to get before I'm ready to enter lol

you can add to her TC once she is entered. That won't change anything as far as her being kicked out of the queue. It is just actually editing her profile that will be an issue. Though pets with more complete TCs have a better chance.