As a larger individual the 12 can support my weight nicely (300lbs). The 12 will also allow for easier tracking because any weight that you have will be dispersed better throughout the kayak. The 10 is nice for maneuverability but slightly less stable with greater weights (you + cargo). So big or small id get the 12. Especially if you'll be fishing or carrying cargo.
No it does not come with a paddle, only on the "Sun Dolphin Journey 10' Sit-On Fishing Kayak"
Super stable! I bring my 70lb lab/boxer all of the time and we have yet to flip the kayak! I will say it is a bit difficult to get her back in the boat once she has jumped out (she loves swimming) however having a life jacket with a handle on the back helps significantly. I would suggest the ruffwear life jacket : https://www.amazon.com/RUFFWEAR-Float-Coat-Atoll-X-Large/dp/B01MZ9ECCJ/ref=sr_1_2_sspa?crid=131W9HX6V1HEX&keywords=ruffwear+life+jackets+for+dogs&qid=1564088954&s=gateway&sprefix=ruffwear+%2Caps%2C214&sr=8-2-spons&psc=1&smid=A1YRA3K6IOXX6E
There is no part of this kayak that is covered and you truly sit on top.