They may get along, they may not, but I have always kept at least one, up to 150 crayfish in my tank (when they breed) and have yet to have a problem. So I think you should be fine, just give the little guy his own place to hide out during the day and for when he molts.
It depends, I went through 3 common plecos, about 3-5"", all within only a few days of putting them in, lost to my crayfish. But somehow he gets along with my 4" bushynose pleco which I have had in with him for a few months.
i was gonna do the same thing but decided not to when my small crawdad caught/killed/ate a 3" goldfish that just happened to go inside his cave.
The fish was a beautifully colored fellow too...