Those who do testing/bugfixes could be the 20% too. Yes they probably will be paid less then core developers but they're still the 20% to keep and get rewarded.
I would say for each job category, reward their top 20% more and do your best to keep them.
You might work for much bigger companies than I have, but there is no 'bug fixing' vs. 'core' job categories. Instead it's like "Susan worked in that area once last year, the expert has left, so Susan can fix it, while the feature work she was on gets postponed a bit".
