Is the Ratling getting the +4/+4/+2/+2/+0/-2 stat adjustments for adding class levels?
Either way, I would have to pick something that benefits from whatever stat you have UMD keyed to. Take advantage of that Scroll Use Ex... so probably Charisma, or Intelligence (with a trait).
Swashbuckler is good. And the Mouser is a fun dip for someone serious about making a Vexing Dodger UnRogue. With a racial Climb speed, a Ratling will make an awesome Vexing Mouser.
If you are going to a high even level with this Ratling... say 15 of them [class levels, that is]...
Mouser Swashbuckler-1/
Vexing Dodger UnRogue-8/
Mouser Swashbuckler-X
You get the Underfoot Assault deed at level one, which is fun and gets you access to flanking for Sneak Attack.
Vexing Dodger is one after another hilarious and awesome features for climbing all over your enemies, stabbing their important organs in alphabetical order. The Dirty Tricks and Debilitating Injury and Distracting Climber can be quite effective.
Continuing back into Swashbuckler for the Hamstring deed gained at Mouser-7...
Nasty little Vexing Mousers.
PS. This is made even more hilarious if you are TWF with piercing weapons and have the Weapon Trick feat for Piercing Climb... there's no rules saying that you do damage with your daggers using them to Climb your opponent, but there's also no rule saying you don't. Lol.
The stat adjustments are from the Advancing Monsters section. I believe it's from the Bestiary, but I prefer the format on D20PFSRD... despite it being less official.
Having a Climb speed and grabbing the UnRogue skill unlock for Climb with max ranks can almost guarantee success for the maneuver, even without Athletic, Skill Focus Climb, or Agile Maneuvers... although all of those definitely help.
Whoever you are climbing takes a penalty to attack you AND a penalty for attacking anyone other than you! It's hilarious.