You can't put more than one +1/+1 counter on any one target using the support action.

"Partner with [name]" represents two abilities. The first is a triggered ability: "When this permanent enters the battlefield, target player may search their library for a card named [name], reveal it, put it into their hand, then shuffle their library."

Note that the target player searches their library (which may be affected by effects such as that of Stranglehold) and that the card they find is revealed, even though these words aren't included in the ability's reminder text.

A nonlegendary creature can't be your commander, even if it has a "partner with" ability.

Support can target a creature another player controls.

If a spell with support has other abilities that target creatures, those abilities and the support ability can target the same creature.

If some, but not all, targets for a spell become illegal, the remaining targets are affected as appropriate. If all of a spell's targets become illegal, that spell doesn't resolve.