Just a update, I decided to hold off until I heard back from wp-engine, their response
We do automatically force some tables to InnoDB for performance reasons. You can change the database engine on your end to create the index, but please bear in mind that it will likely be a temporary change. I wouldn’t consider this to be a bad thing, as the only consideration is from a performance side, not server stability or security.
My response back was why would it be a temporary change? Haven’t heard back yet, so since they gave me the green light, I’ve tested switching it on my staging server and it works, it works with and without creating the index. Is the index necessary?
One other weird thing I noticed, every table for simplepress is already MyISAM, the only two that are InnoDB are the sfposts and sfoptions tables, is the sfoptions being InnoDB a concern?
I did create the table and tested it, search still works fine, just want to double check this is the way to do it if needed to create it before I do it on my live site, thank-you

