Will Mangler have instances when population gets too high?

The description it says Mangler will be standard when it comes to instances and rares. Does this mean no instances? If 200 people are in Lower guk, we'll just have to fight it out?

It's in the Mangler FAQ:

Q: How will you be handling the rush of players at launch?
Zones have a form of load-balancing that can spawn another version of a zone when the current zone gets too full.
Once those zones reach a certain threshold of players (which varies based on the size of the zone), it will spawn another version of itself. You will be able to choose which version of the zone you want to enter, or you will be able to use the /pickzone command to choose another version of the zone you are in. This should let people group up with friends while still being able to find things to kill and interact plenty of other players.
We also utilize technology to dynamically increase the maximum number of characters allowable on a server at any given time. Between these two systems, we expect to be able to handle the players who want to play on the new server on the first day.

They'll have *pick zone* 1/2/3/4 etc I imagine.

It will have pickzones that spawn when a zone reaches some # of players. What I take 'standard' to mean here is that the Agent of Change lockouts are the same as Phinny/Coirnav/Agnarr

Pickzones (instances) like other recent TLPs.

"standard when it comes to instances" is referring to DZ lockout timers. Just means the standard lockout for AoC instances will be 6.5 days rather than the accelerated 3.25 days that Selo has.