MyISAM with Innodb in the same Wordpress database reduce performace?
Having InnoDB won't make MyISAM slower and vice versa. However you have to be more careful in memory allocation for InnoDB and MyISAM. Because they store data and indexes in different buffers.
MyISAM:


Indexes in the key buffer
Data in file system cache


InnoDB:


Data and indexes (well, in InnoDB everything is indexes) in the buffer pool


So if you allocate more than necessary to InnoDB then MyISAM will have to access disk more often (and vice versa)