Thanks for the answers!
I pulled the brown head at 48 hours and did another full skim at about 72 hours on a recent batch. It felt like I was removing an awful lot of great-looking yeast from a beer that was still fermenting. If I get diacetyl or poor attenuation from the donor batch, next time I'll leave a good portion of the second head behind and probably mix it back in. Now that I know how little is actually needed to pitch a new beer, I won't feel the need to take the whole head anyway.
Thanks again.
For any other beginners like me reading this, if you can plan your brews accordingly, top cropping is not only the easiest pitching method (versus rehydrating dry yeast or making liquid starters), but it should also yield an extremely clean, viable pitch. For example, I recently made a 1.045 batch, a 1.055, and a 1.065 batch in succession, each about 4 days apart. I pitched the wyeast 1318 top crop from each batch into the next, and it allowed me to pitch a large amount of healthy yeast into three batches after making only one small starter for the first beer. Less starters, less work, less chance for contamination.
Of course, the catch is that you have to like top cropping (usally English??) strains. Thankfully I do.