You must unlock every costume and character to choose multiple people (i.e. you can choose clones of the same person, even for both players).

Exit and re-enter the shop screen until the price of the extra fighters are lower.

To change the time on the clock in the clock level, simply change the console's system time.

To get lots of points for buying characters and such, simply go to practice mode and chose any characters. Then, leave it there (without pausing) when you won't be able to play for a while and turn off the t.v., but not the console. Depending on how long you were gone, you could get as much as 7,000 points.

This is a handy tip for saving more points so you can unlock more stuff: If you go into the shop and he costs 2,000 points, simply exit the shop and re-enter, if you see Mega Man again, he could be reduced to as much as 500 points.