Minimum rowcount for filtered index?
For example, if you have 100,000 rows in a table that gets, say, 75%
  read operations, would it be wise to add a filtered index that only
  covers 500 rows? Or 100 rows? What if the filter covers 85,000 of
  those rows?


That would depend on the queries against the table. If query patterns are such that they target a subset of the 500 or 100 rows that your filtered index covers, perfect. On the flip side, it's unlikely that the optimiser is going to choose a filtered index that includes 85% unless it is a covering index for a particular query.

I haven't tested this but I would expect filtered index utilisation to exhibit the same tipping point behaviour as non-filtered indexes.