Better trying first (playing out of position) in one-two non important games, but for my stats :
a DC, DL, DR can play also good in anther position, so I have DC-DR & DC-DL (to switch from narrow to wide if necessary).
Try a DC or MC for DMC position. Usually working.
MC for ML or MR. Well, it's hard to find a functional MC, not to add one more position - lol.
MR (or ML) >> AMR or AML usually working.
AMC/AMR + AML, all around AM. Usually working.

Special abilities - one of the unanswered mysteries of t11 - lol
I don't think anyone can answer for sure about function of sa but I use to set some of them
GK > one-to-one
Defenders > aerial or defensive wall
I always try to give those sa. I had during my career good defense and low avg of conc.goals but I don't know what the part of sa for that.
Sometimes I give free-kick in some Mdf or attackers.
It working sometimes, for some players, for some periods - lol
* but when I 'm buying a scout/list MC, I prefer to take my chance with this sa.
I tried some times sudden striker for wingers. They performed good but still don't know how % it's the participation of the sa.
One-to-one for my ST. It seems that it's working some times.
I don't have any solid conclusion about corner specialist.
Penalty specialist, dribbler, play maker, were invisible to me.
Usually I 'm buying nordgen 18 y.o. and keep them for two contracts, 6 years.
An extra special ability for a 18 y.o. good trainer it's about 60-70 greens (+ reds for 1 injury avg). So if I keep a player for 6 season, it's about 10-12 packs/season. I think it's worthy. Also it's easier to sell this player later.