Martin, the fish you photographed is definitely not filamentosus. It’s a male and it is not too young. It’s appearance shows markers of a male P. spec. of about 7 – 10 months. Male filamentosus show a very elongated caudal with a clear black filament rather early in their life. When very young, the filament is very small, but the fish you showed at the picture should have that typically coloured elongated caudal with the tyoical black filament. Look at the typical colour of that caudal; it’s a very constant marking. (The picture I add here is a famous shot by Hans-Joachim Richter from the beginnings of the eighties of the last century).
But maybe there are real filamentosus mixed in the group you have bought, and still smaller. You can only wait and see. Then we will perhaps recognize waht the species of that fish is what you showd to us.
And I understand your difficulties: I shall bring no fish with me when I visit Poland in May. Later this year you will come to Germany and take fish home.