ElRey,

Changing the fork on a given frame will completely change how it handles, even to the point of making the bike unstable.

Of course all other factors are considered but the frame's steering angle and fork offset are primary considerations.

And again I will state that with tandems, with very few exceptions, the frame geometry is the same regardles of it's intended use.

And here are some long-establisthed general guidelines for single bike geometry:

Racing frames:

Crit racing bike - moderate to steep head and seat angles, short trail, high bottom bracket for quick steering and pedaling through corners.

Road racing bike - moderate to slack seat angle, moderate head angle, moderate to long trail, lower bottom bracket for comfort, more control descending, powerfull climbing position.

Road racing TT/Tri bike - extremely steep seat angle, moderate to slack head angle, moderate to long trail, low bottom bracket for aero riding possition and stability.

Through the size range:

Smaller frames have steeper seat angles and slacker head angles and more fork rake. Larger frames are the reverse.

Small changes in rake/trail - 3 mm - can make a big difference in the way the bike steers.

Due to the advent of brake/shift levers, racing bikes in general have less trail than they did 10 years ago IMHO.