#rules citation

[magicjudge]

If a spell causes you to proliferate, and there are no petmanents/players with counters on them, do you still proliferate? Asking for the new cards that care about you proliferating, like Venser and Ezuri.

701.27a To proliferate means to choose any number of permanents and/or players that have a counter, then give each one additional counter of each kind that permanent or player already has.

As you can see from the definition of proliferate above, you get to choose any number of players or permanents with counters, which includes zero.

Since you're allowed to choose zero things to add counters to, doing nothing when you proliferate still counts as proliferating and will still cause cards like Ezuri or Venser to trigger.

[magicjudge]

I am trying to help a few younger players . One of them was trying to understand Planeswalker interactions with the stack. This situation arose: Player plays and resolves Arlinn Kord. Opponent responds with to the slaughter. I said (since it was a non competitive match) that he could have still used Arlinn's loyalty ability before he passed priority. Can you please show me the rule where this is established from the comprehensive rules? Thanks for the help!

The rule you’re looking for is part of rule 116. Timing and Priority: “116.3b. The active player receives priority after a spell or ability (other than a mana ability) resolves.” This spells out the basics of the situation, giving priority back to the active player. 

To get the full picture, though, we need another rule from 606. Loyalty Abilities: “606.3. A player may activate a loyalty ability of a permanent he or she controls any time he or she has priority and the stack is empty during a main phase of his or her turn, but only if no player has previously activated a loyalty ability of that permanent that turn.”

Taking these two rules in combination shows us that the active player will have priority after a planeswalker resolves, and since it will be that player’s main phase and the stack will be empty at that point, that player will be able to activate a loyalty ability of that planeswalker.