I had to download Google Keyboard from the play store and install it. So Breadwallet deems this to be a "Bad Thig™" and refuses to work with it... it wouldn't matter if I was using Flexy or Swype or anything other addon keyboard.
Android already gives you warnings that addon keyboards can steal your data etc... and as long as a person is sensible about what they're doing, and trust the company that coded the keyboard, I don't see why Breadwallet should have an issue with it.
It may be a bug. It should only disallow custom keyboards when typing in your recovery phrase, since obviously this is extremely security critical and you don't want your keyboard remembering what you type and adding it to auto-complete or sending it to a cloud service for improve typing prediction. O.o You should be able to use any keyboard you want any other time. If that's not the case, please report to [email protected], or submit an issue to https://github.com/breadwallet/breadwallet-android/issues