Idk about how the geos and angels would work. But with enough filtration and water changes regularly it could work. I have a 135 with a heavier bio load because I have 2 O's and my stock is fine with 2 50% water changes every week.
I agree. Congo Tetras get large and that amount will upset your bio load and also add a lot of clutter since a 125 is a shallow tank.
Why not try a SA Tetra? Lemon Tetras come from the same waters as Red Heads. Any of the "Rosy" types would work too and allow you to have a decent school numerically.
Lemons won't get as big as congos, don't cost as much and will still school just as tightly.
I asked almost this exact question here a few months ago with 8 gel red heads, 6 angels and a school of 10-12 lemon tetras. I got all agreements from posting members. I altered my stock and now will have 6 geo red heads, 2 keyholes, 4 angels, and 2 super red sevs. No dithers unless I need them and they will only be 6-8 lemon tetras.