The Free Archetype variant rule does not account for all possible rule interactions, so a lot is left up to your DM.
Notably, if you take an MC dedication at 1st level, and if you are using the Free Archetype rules, strictly speaking you can't take any archetype feats at all at 2nd level, since all the follow-on feats are specifically 4th level or higher. So your DM is going to have to take a stand on how to adjudicate it. There are basically 3 ways to go:
1) Avoid taking options like the Ancient Elf heritage when you're playing with the Free Archetype rules, since you'll have access to archetypes very soon, at 2nd level).
2) Your DM could exempt the MC dedication you took at 1st level, and allow you to take a different archetype normally at 2nd level, bypassing the 3 feat rule for archetypes. This is probably the best solution.
3) Your DM could allow you to take a 4th-level archetype follow-up feat at 2nd level, so that you don't have a wasted feat slot, or else allow you to take an additional normal class feat at 2nd level, since a strict reading of the rules would make it impossible to take anything in that first Free Archetype slot. I don't think I'd choose to do it this way, but some DMs have.
Note that although a lot of people give absolute freedom in selecting Free Archetype choices when they decide to use that variant, I believe it was originally intended to only allow a limited selection of archetypes (sometimes only one) that are appropriate for a given campaign. You see this in the limits placed on Free Archetype use in the Strength of Thousands AP. So your DM can do it whichever way he likes, totally open or highly restricted, or somewhere in between.