Free seating is NOT an alliance perk in Skyteam.

The individual airlines may offer free seating on all ticket types to their own (top-tier) status members, but even this is not guaranteed. While FB Platinums can pick any seat for free at any time, for Gold/Silvers this only kicks in within certain timelimits prior to departure, and in many cases will only give a discount on the preferred seats - not all seats are free.

Skyteam is NOT the alliance for you if you want free seats for you AND for all your travel companions.

Perhaps you should just consider using the FFP of the airline you use most frequently. This is - in almost all cases - the best choice of FFP.