There is a problem with the way that REDUCE mode handles an exclamation mark (‘!’) followed immediately by a double quote ‘"’. See Font-lock support for automatic font selection. This should not be a problem in “normal” code, but it may upset the parsing of code that uses this character sequence within an identifier. It is caused by a limitation in the way that Emacs currently handles the syntax of the text being edited and is not easy to avoid completely! I am looking for a better resolution of this problem.
There is no guarantee that an arbitrary minor mode or other extension will be compatible with REDUCE mode, although I am not aware of any conflicts. Two minor modes that are known to be compatible with REDUCE mode are transient-mark-mode and delete-selection-mode (because I always use them and I recommend them!).