Hello! Welcome to the forums. I hope you enjoy your stay. As to your questions, I'll try to answer them in order:
Double posting is discouraged, but it's acceptable for bumping threads, or when you're using a phone/other device that doesn't play nicely with the forum software.
Dwarves can, in theory, use weapons in two hands. What you're probably reading about are pikes, halberds, mauls, and a couple of other weapons. The minimum size for a creature to use those weapons is slightly higher than the size of a dwarf, so dwarves won't pick them up. You can't make any of these weapons by default anyway, so you shouldn't need to worry about it.
In the current DF version, there are a few bugs that cause massive amounts of lag, especially in older forts. A lot of them have been fixed recently, but simulating the entire world in the background still tends to slow stuff down.
Things to watch out for include longer history, higher numbers of civilisations/sites, larger world sizes, and larger embark sizes. There are lots of threads on the forum about keeping framerate up, along with the
wiki article.
Population is another big one. Fewer dwarves means more FPS, but also fewer hands to get work done. Play around with population caps and see if you can strike a comfortable balance.
During gameplay, if an army attacks another site, the attacker will always win. Because goblins are more aggressive than dwarves, this means that they tend to wipe dwarves out pretty rapidly. It shouldn't affect your game too much as far as I know.
Embark size is totally dependent on your personal preference and your computer. Some computers run DF better than others, and again, there are loads of forum threads discussing this topic, so I won't go into it here.
I prefer 3x3 - 4x4 myself, seeing as my computer can't handle anything bigger. It sounds like yours can, though, especially if you've got a 9x9 (!!) map and not seeing much slowdown. Just remember that larger sizes will have a noticeable effect on your long-term FPS, so you might want to shrink it a bit. Again, this is totally up to you, so just experiment and see what you like best.
If I neglected to explain anything, or if you have any further questions, I'm sure either myself or another of the fine fellows on the forums will be happy to help you.