I have my Midas with a jag,dovii,umbee,bellycrawler pike,there is slight bickering once in awhile,but i keep alot of open space so no one can really claim thier own territory.
I keep my 9" Midas with the following
1 2" Jack Dempsey
4 5" Red Hook Silver Dollars
1 4" Regular Silver Dollar
1 8" Sailfin Pleco
The Midas was raised with tank mates his whole life so he is a bit milder than most. But he is still king of the tank. I recently added the little JD and he paid no attention to it and does not even bother it at all.
I have noticed that the larger the tank the more accepting the Midas tends to be. When the fish were in the first grow out (65g 36x18 footprint) there was a lot more issue. In the 120g there are a few issues but relatively peaceful over all. These guys will go into there permanent home with in a few months (210g 72x24 footprint) so Im sure it will help resolve any stress there is now.
Yes see I have a baby jag and a baby gt in my 125 so I wanted to get a baby midas and raise them together and like I said I will be upgrading to a 240 in the next few months.