I have just finished watching Flushed Away and there's something you two have missed in trying to decide whether or not Spike reformed. And that is...
The movie ends before that question is even asked.
The last two times Spike and Whitey are seen, they are doing the following:
Attempting to capture Roddy and Rita again.
Observing the launch of the Jammy Dodger II.
Their last on-screen official act as criminals occurs when they are still employed by The Toad and are following his orders to capture Roddy and Rita again. They are not able to do so.
Jump to sometime in the future, and they happen to be around to witness the launch of Rita's new boat.
So, what happened in between those two times? Did they stay in the employ of The Toad? Did they quit? Did they get fired? Did they find themselves out of a job because The Toad took Le Frog's advice to "give it a rest"? Did something happen that convinced them that being criminals is something they needed to stop doing? Were they on a new assignment that just happened to put them in the vicinity of the docks at the time of the boat launch?
We don't know because the movie ends.
In order to say a character reforms, we have to see them reform. In order to say they didn't reform, we have to see them continue their past behavior.
We do not see either of these with Sid and Whitey because the movie ends. As far as we know, it's pure coincidence that they're around when the new boat is launched. We have no context as to why they are there.
Any assumption as to whether they reformed or did not reform is exactly that: an assumption. If you saw me in a library reading a book on human anatomy and then you never saw me ever again, would you assume I went on to become a doctor? Without further interactions between us, you'd have no context and no proof that that assumption was correct and you also wouldn't be able to tell if it was wrong. It could just be a case where I was bored or curious and had nothing to do with any kind of career I may or may not be pursuing.
You two need to stop bickering over this point because there is not enough information in the movie to make a decision one way or the other. Neither of you is right. Neither of you is wrong.
Get over it and move onto more worthwhile things like the other 99% of what happens in the movie. There's one sentence on Sid's page. There's one sentence on Spike's page. There isn't even a page for Le Frog. You've got an 84-minute movie. Surely there's more than two sentences for three characters in there somewhere and there has to be more than fussing over a minor point like "did he or did he not".