The mod should be able to insert equipment to player automatically
(with checkbox for those that didn't want the functionality).

It's for convenience.

If you implement this, and thinking this might be difficult because the need to check a free slot in the chosen armor; I don't think it's necessary to check armor equipment "free" slot, or even how much it can hold.

Just force feed the chosen entity to the chosen armor, if failed, give error console message to player, and the failed entity will be put in inventory instead.

Those that use a quick start mod and picking startup item know very well how much this X type of armor can hold X number slot.
Or how much slot this X type of equipment will occupy.
So the user error will not even that high IMO.

Of course there would be an edge case where a mod altering the number of slot in armor,
Or make the equipment slot size smaller,

For that issue, well i don't know how you can resolve it.

Add to that, +1 for the ability to remember the setting that someone suggested 1 month ago.