I want to know how many drop tunings I can reasonably use with one set of strings. Ex. can I go from E standard to drop C on 11s? For a given size string, how many tunings will be useable, given that E standard has to be the highest one? In the best case, I would like to go down to drop B or drop A. Is that feasible?
It is feasible, yes. Your challenge will come from the low tension you will require on that bottom string. Using standard strings will mean a much lower tension than if you use thicker strings.
Effects of lower tension include:
Going down to a drop A is a pretty major drop, so I would suggest that unless you like the slappy type of sound and are very light with your fretting hand you will want to go up one or two gauges on this string.