Might not handle exactly the difficulties you're having but you can do
`from sympy.abc import *`. This will create symbols for pretty much all letters (lower and upper) as well as a bunch of greek symbols (but written out, e.g. "delta").
Obviously you could find a list of every ticker symbol and create a few thousand symbols before parsing, but you don't always have the luxury of a complete/up-to-date list, or doing so might create too many symbol objects and cause performance problems.