Street Fighter X Mega Man/Secrets

No Helmet Mega Man
During the Stage Select screen hover over Ryu's icon and hold the Shoot button then press Right, Right, Right, Left and Mega Man's icon should be helmet-less.

Change the Music
In any stage enter the the pause menu and hold the Jump button and press Up, Down, Down, Down and the music should change to "Goes With Everyting" which is Guile's theme from Street Fighter II.

Hadoken and Seiei Enbu
During the title screen hold the Shoot and Jump buttons for 6 seconds and a chime should occur.

Hadoken
Mega Man is able to shoot a hadoken just as Ryu would in Street Fighter by pressing   (or quarter circle  and the shoot button).

Seiei Enbu
While in a Stage press the Shoot button with the WeaponSwap1 button at the same time and you should hear a chime confirming that you did it correctly. Transparent Mega Mans now follow the real Mega Man and do as he does but only the last one is able to shoot.

Gouki/Akuma
Get a Perfect on 3 boss fights before the Teleport System and during the battle with M. Bison (Vega in Japan) Akuma will come out of the sky when M. Bison is almost defeated. Mega Man will now be teleported to Akuma's lair to fight.

Tip: To make this easier simply use an E-Tank to recover your life when you are about to defeat a Boss.

Sagat
Get a Perfect victory against four bosses in the Teleport System, and before the battle with the Dictator (Vega/M.Bison), you will fight Sagat. Beware that if you lose against the Dictator, you have to fight Sagat again, too.

Sagat is available in V.2 only.

More E-Tanks
The passwords can save up to 4 E-Tanks.

To get more, Mega Man should collect the four E-Tanks in the initial stages (Ryu, Chun-Li, C. Viper and Dhalsim) without beating them, then beat a different Street Fighter to get a password and reset the game (either F12 or Esc). By re-entering the password, Mega Man will keep the four E-Tanks, and the stages will still have four more E-Tanks.

This allows for easy Perfect victories: feel free to use one or two E-Tanks in each of the four first boss battles (Blanka, Rose, Rolento, Urien), then get the password and reset to collect up to 8 E-Tanks at a time.

As a consequence, it is possible to unlock Sagat and Akuma in the same game. Once you get three/four perfect victories in the first eight stages, stock up 8 E-Tanks. In the final stages you can find two more, for a total of 10. Use up to 4 of them to get 4 Perfect victories in the Teleport System, and you should be left with 2 E-Tanks for each of the final three bosses (in order: Sagat, the Dictator and Akuma).

The password system is available in V.2 only.