By law, every steward is required to do a qualification (NVQ) in Spectator Safety. That is the training.
Plus for searching, chucking out etc.. SIA Licensing is in place. Same license as any doorman.
It doesn't matter if it is your fault or not in these situations unfortunately. Certainly other stewarding companies are actually instructed for anyone on track/park to not be escorted out but actually be arrested. I would suspect SecuriGroup are the exact same.
I'd ask the club about it. I know at other stadiums in Scotland flags are allowed aslong as they are proper flags and not just bedsheets, and are on garden canes.
The problem is that the stewards need 2 people to see it so any charge could hold in a court. Same with the sectarianism and racism else if you goto the punter then it is deneyed straight away.