If you just want voice control of the Arlo, you can do that with just the echo, the Arlo, and the free IFTTT service. You don’t need to involve smartthings at all. 
The Arlo channel has options for arming, disarming, and start recording:
Alexa devices have built-in IFTTT integration. So you use that IFTTT channel/service to define whatever phrase you want to say. The only thing is that you have to say “trigger” to start the phrase so that the echo will know that it’s an IFTTT command.
So you can create an IFTTT applet using Alexa as the “IF” and Arlo as the “that.”
After that, when you say something like
“Alexa, trigger the front porch camera” then the camera would start recording.
Or
“Alexa, trigger arming the front porch camera”
Or whatever verbal phrase you want, it’s just that you have to start each time with “trigger” so Alexa will recognize it as an IFTTT request.
SmartThings also has an IFTTT channel/service , so you can use the same “trigger” format with it. But it also has a separate official echo integration which means you don’t have to use the trigger word. However, in that case, the only options for an Echo-recognized SmartThings device are “turn on/off/dim.” Or “set” for a thermostat. So “Alexa, turn on the front porch Light” or whatever. There isn’t anything more to it than that. From Alexa’s point of view, you’re basically just turning things on and off.
https://support.smartthings.com/hc/en-us/articles/207808076-Amazon-Alexa-SmartThings-FAQ
Where the magic happens is that smartthings allows you to create a “virtual switch.” And you could have that switch turning on trigger anything that smart things can do, including Mode changes, arming the SmartThings security features, unlocking the door, etc. Echo still thinks it’s just turning the switch on and off, but smartthings recognizes that switch coming on as the initiator for anything else that the smartthings system can do. And a virtual switch doesn’t exist in the real world – – it’s just a bit of code associated with your account. So you could have dozens of them if you want and they don’t cost you anything.
So lots of different ways to do things. But if you just want voice control of Arlo, I think I would look at the IFTTT options first. And with that method, you can make the verbal command any phrase you want, it just has to begin with “trigger.” The actions you’ll be able to initiate with that phrase are defined in the “action” part of the IFTTT service/channel for that particular device.
Oh, and here’s the FAQ for creating a virtual device if you want to start playing around with those: