MIMF

Can I use cherry or black walnut when making a bridge? Trying to use local woods. I have some scrap pear, can that be used?

Pretty much any of those woods would work. However, if you are building a steel string, you might want a little bit more bridge weight than you'd get from Black Walnut. I don't do steel strings, but my impression is that a bit heavier bridge is good for sustain. Personally, I'd think Cherry would make a fine steel string bridge, where it might be a bit heavy for a classical bridge, or, at least, my classical bridges.

Either walnut or cherry would be fine. I like to make the bridges a little bigger when using a low density wood. This gets the weight up into the 'normal' range, and gives you more gluing area so they're less likely to come off.

Al, I'm guessing that making a bridge wider rather than longer would be better in terms of helping the bridge stayed glued but I haven't really hashed it out. Thoughts?

Pear would have the desired density and a long history in acoustic instruments as a fill in for ebony and other exotics.
Other very hard domestics you might want to consider would be Osage orange, mesquite and "mountain mahogany".

Wider (from front to back) is certainly better than longer. The maximum shear stress along the back edge is a function of how long the glue line is along the line of pull, so making the bridge wider reduces that. Once the maximum stress along the back edge is exceeded the thing will come up.

Martin started out using a 'pyramid' bridge that was abou six inches long by an inch wide. When they went to steel strings as an option they had too much trouble with them, and added the 'belly' along the back edge, which reduced the problem to manageable proportions. The total footprint area of a Martin bridge is still smaller than that of most Classical bridges, but because of the width they stay down pretty well, where a Classiacal style bridge tends to pull up if you use steel strings.