I cant see it being an issue personally, any fish can fight but they are extremely different colors and species from eachother, the Midas blennies stick to the high water column and rocks, the Bella goby sticks to lower water column and the sand bed generally. Ensure to quarantine the new specimens prior to adding, but it should be fine. They dont compete with eachother for anything other than during feeding times where Bella’s can be a bit pushy, but the Midas is a “snatch and run” type of eater when aggressors are in the tank.
Just my personal experience with both species, I’ve never attempted to house them together but have kept diamond gobies with Midas blennies with no issues in 20gal cubes (store tanks).