Personally I say Yes it is Milsim enough.
What Defines Milsim?
I have been asking this same question.
I run a tac one with a stick feed, or a pump with a stick feed...
My argument is the look of the marker is not the ONLY criteria to make up milsim. The game, the play style, the limited paint, the objectives, the scenario...all play far more important rolls in milsim than just the look of the gun...
So if your friends are worried about the Tac One, what are they using? Pure Mag Fed markers only? Or do they use a tac cap or other loader?
Use the Tac one.