Jubilant Mascot

• WWizards of the Coast2018-06-08
  Official Ruling
  Support can target a creature another player controls.
• WWizards of the Coast2018-06-08
  Official Ruling
  If a spell with support has other abilities that target creatures, those abilities and the support ability can target the same creature.
• WWizards of the Coast2018-06-08
  Official Ruling
  You can't put more than one +1/+1 counter on any one target using the support action.
• WWizards of the Coast2018-06-08
  Official Ruling
  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.
• WWizards of the Coast2018-06-08
  Official Ruling
  You choose the target creatures to support when Jubilant Mascot's triggered ability is put onto the stack, but you don't choose whether to pay until the ability resolves.

Rulings via Scryfall