Keep in mind, as Roger indicated, the size of the bolt and what it is threaded into, tighten it as you think appropriate. If it gets loose, tighten a bit more next time.
I think the bolt that does the most to hold the alternator in place is where the alternator is bolted to the adjustment arm. You are not likely to over tighten that. The bolt for the pivot for the adjustment arm goes into the aluminum bell housing so you should just get it good and snug rather than try to get it really tight as you'll run the risk of stripping the threads. I could easily be wrong on it's purpose, but it seems to me the long quarter inch bolt at the alternator pivot doesn't seem to provide a whole lot more than some stability. Given that you are tightening the bolt into a nut, you aren't likely to over tighten that but the sleeve in the alternator can dig into the soft aluminum of the bell housing so you shouldn't get overly enthusiastic there either. Make sure it's tight but it's not critical. You might consider putting a washer under that sleeve to help protect the aluminum.
What's the definition of "tight" or "really tight" or "snug" or "good and snug?" It's really subjective but my point is that the exact tightness of those bolts isn't all that important. You just don't want them to come loose.
