I agree, gamma is bad here because it is not general enough.  But 'p' and 'p+' *are* the chemical symbols for proton.  This make it consistent with 'e' and 'e-' for electrons and 'n' for neutrons.  Photons need to have have a different non-ambiguous symbol.  I don't really care what this symbol is as long as it isn't 'p'.  Since this is just a naming convention for tags, why not spell it out more.  'phtn' and 'photon' would both be acceptable.