Yo! Glad your taking up 2a! Brace yourself man, it’s insanely hard.
Now, regarding your questions:
I personaly have not yet tried the Firedogs, but I expect no less than great from YoYoFactory. So, yes, I’d say they are good, but just in case they’re not that great, you could try Loop 720’s instead. Now those are some good loopers!
Yes, shorter string is best, but you can use whatever lenght you like best. I always cut off about 2-3 inches from a full lenght string, and that works best for me, but feel from to go longer or shorter if desired
Thin string is usually not graby enough for quick response during looping tricks, especially when your just starting out. I prefer 50/50 cotton and polyester string for looping. The thickness of cotton, with the durability of polyester.
If you have anymore questions, feel free to send me a PM if you’d like.