Pictures will hopefully follow soon :-)

Wanhao i3

No

Doesn't Matter

Supports shouldn't be needed, though the torso with the arms might benefit from them.

I don't recommend printing the full size, I printed it rescaled to 0.6 size, and that turned out to be quite large enough (horses are big :-).

The connector bolts are to be printed as one, then broken apart. You may need to remove the extra bits that hold the bolts together.

The third source isn't on thingiverse, so here's the link:
https://www.tinkercad.com/things/96OOl1LbrRK-centaur

At first I hoped to be able to change the centaur-part in a way to have the minion just "drop in", but the connection never looked real and the arms got in the way.

So I merged them together, smoshed the originals arms away in Meshmixer and added in the beefy arms.

The head portion is cut at an angle to make sure the eyes/goggles are printed correctly and without support.
If you find a head without neck that is wide enough, you probably can make another Centaur from it :-)