Rikudou was born from a Goddess (more or less) meaning he already pretty much had special powers enough to fight and seal juubi, i personally think he had sage mode for that ( but its more speculation than anything , he discovered ninjutsu and went around teaching it , it wouldn't be a far stretch to say he also probably discovered sage mode (as he was referred to as rikudou sennin) , i know its mostly speculation but it depends whether he got the name before or after sealing the juubi.(almost everyone called a sage has had sage mode)...i do have another speculation that rikudou probably had something to do with all the sage animals ( frog,snakes ect ) why because i believe one of his bijuu is also some kind of sage and i believe someone like rikudou would have probably discovered this.
At the end of the day besides his name i cant really give much of an argument without speculation but in the past speculation has proved useful as most predicted hashirama had sage mode (while most found it funny i might add) ...most things in naruto are kind of obvious and just need common sense.