Just wait a few dungeons.
After you have used a Pokemon that is higher level than you and your partner Pokemon, then that Pokemon will sometimes not be usable for the next few dungeons. This is because otherwise you could use that level 50 salamence over and over and destroy everything, so they had to balance it is some way. Sylveon and Chansey should be useable again in a few dungeons, so don't worry about it too much, and simply replace them for the time being.
Hope I helped.