It's all basically true. Rockman was considered for release on an arcade platform as well as for the Famicom Disk System that had come out the year before. This was mentioned in the Rockman 1~6 Soundtrack booklet, as well as the Rockman 7 & Rockman X3 Cho Hyakka source book.
The Astro Boy thing was a little different, Capcom never actually had the rights to Tetsuwan Atom, but they wanted to get them because they thought a game about Atom would sell better than a game starring a brand new character. For that reason they first planned the game in a way that once they got the rights, they could easily insert Atom into the lead spot. They never got the rights, so it was Rockman's debut instead. This was mentioned in the interview portion of the Rockman 10 years History Book. Japanese Wikipedia says that Inafune also talked about this in the Capcom Consumer Chronicle vol 2 DVD. I don't own that, but the other sources I can attest to.