Ho to report a bug?
First look around. See if the bug is not already reported on the official tickets list, or if it’s not just a common problem. If not, read the advice on this thread to learn what data is needed to properly report. After this, you best bet is probably the bug report section of the BuildCraft forum.
Can I use BuildCraft in a mod pack?
As of November 1st, 2011, the answer is YES. But that comes with constraints and restrictions. BuildCraft is released through an Open-Source license, the Minecraft Mod Public License. It does allow redistribution of the mod in its various forms (sources, binaries) in certain cases. Make sure that you do understand the license before distributing a mod including BuildCraft!
The main restriction is that sources must be accessible. If you are only redistributing the unmodified binaries, or class files, adding a link to http://www.mod-buildcraft.com is enough – sources can be retrieved from there. Otherwise, make sure that you clearly understand the implications, and asks questions in the forum if needed.