The threadfins would not eat anything bigger than a baby brine shrimp. Maybe a tad bigger but basically only very fine food. The Microrasboras on the other hand completely ignore small foods like brine shrimp. Also the threadfins eat from the area close to the surface and then go to the bottom. Microrasboras eat from the bottom area of the water and also from the bottom. So these 2 species will be fine.
The gertrudae is a little different because it does not act as prissy as the threadfins (staying more at the top and not really wanting to eat from the bottom). But the Microrasboras are not really the shy, tender fish that we all think it is. It does not stress easily, and I have to say it can endure weeks of hunger if it has to.
Overall the combination of these 3 fish is going to be very, very nice. I'd say the tank has to have more of a "spring" felling - bright, happy etc. Lots of stem plants, but also a few dark areas for the Microrasboras to hide when they want.