The Magma is a 510 and you can use 510 accessories on the attys. The threading is the same.
From my understanding, the cart design is a bit different from the Joye 510 carts. For one, they apparently hold more juice. My only 510 is my Magma, so I can't say for certain.
The Magma carts are easy to mod using blue foam, Pyramid Tea Bags, Buff Puff, whatever. Check the Volcano forum for ideas from other Magma users and poke around the cartridge mod section of this forum for ideas. Just about anything that has been tried can be tried on these carts.
Also try dripping once you have ordered juice from somewhere, just be careful since you have auto batteries. The magma manual batteries should be for sale in the next week or so, or pick up a 510 manual from a different seller ( I got one from Litecigusa.net and love it).
As for juice, you really should just start ordering to find out what you like. If you don't mind the chinese made juices, you can find them cheap a lot of places. After trying American mixed/made juice, the china made ones just seem nasty to me. American made/mixed is slightly more expensive, but if you like it and will use it then it is worth it. Eliquidplanet and volcano are teamed up, and you can get 15% off by clicking on the eliquidplanet banner ad on the Volcano homepage. I haven't tried them yet, but if you want to get a sampler pack and try american made in the same order they carry those items. There is a trade section on the volcano forum and here if you order something you don't like.
There really isn't a best flavor or supplier, if depends on your tastes and needs. I've ordered from MyFreedomSmokes, Litecigusa, Johnson's Creek, Liquidexpress, strictlyejuice, and just today from tastyvapor. They all have their strengths and weaknesses, so keep trying new things until you find what works best for you.
welcome to vaping!