UsernameIsTooLon

First time using forge outside of FTB, I'm trying to mod vanilla. I've read through the EAQ and the recommendation for the error I'm having (forge not launching minecraft) is to update forge. But I want to run 1.7.2 mostly because the mods I want are only updated so far. There's probably something really simple I'm missing here, but, how are people supposed to play older mods in this case? I *did* run forge for 1.8 and minecraft launched. Deleted .minecraft folder. Ran latest 1.7.2 build and started having this problem. Deleted .minecraft and tried it with recommended 1.7.2 build; same thing. Anyway here's the log: