Are there short versions for the 3rd person, masculine, feminine and neuter etc. 1st person plural, 2nd person plural and 3rd person plural?
there are short versions of 'jemu' (3rd person masculine dative of also 3rd person neuter dative of 'ono')) - it's 'mu': Powiedz jemu = Powiedz mu.
for accusative and genetive there is 'go
' which often replaces 'jego
' (one can say 'go' is more frequent actually - one caveat 'go' can't replace 'jego' as a possessive pronoun) ('jego' works for both masculine and neuter gender for genetive but not accusative - the accusative of 'ono' is 'je': Widzę je.)
Nie słuchaj jego. = Nie słuchaj go. (genetive) (Don't listen to him/('it'))
Widzę jego (sounds strange) = Widzę go. (sounds normal) (I can see him /(it))