reduce to one cavern, small embark, temperature off, avoid rivers and brooks. If you want infinite water at surface level, become familiar with aquifers, as they provide infinite water for traps and other uses, as well as (to my knowledge) allowing easy to construct, low-fps-cost water wheels, by having one level of aquifer constantly drain into another.
Otherwise, I would suggest you use the limitations of your netbook to give your fortresses direction in setting goals for yourself. There's the classic hermit fort, or you can go starting-7 only and atomsmash/drown the rest. This might lead to ghost-fort but ghosts might cause pathing/lag issues (might no longer be the case). These forts lend themselves to no-military, airtight defense, and fun with ridiculously effective powerful traps and multi-layer security systems. They also lend themselves to multi-talented highly trained dwarves, working in fortresses that don't need much space and are designed in meticulous detail because of the low space requirements.
Defense:
Due to fps concerns and lower hauling capacity, i might combine these traps with atomsmashers for quick cleanup. Dodge-trap where the landing zone is all atomsmashers, or force invaders to path near the magma sea and make them dodge into magma. You can deal with migrants by blocking their pathing to your fort and putting a meeting zone near an outside atomsmasher.
Migrants:
If you don't want to kill all your migrants, but you also don't want them to a) have a lot of space b) build a large fortress c) have a lot of stuff. I might suggest prison-fort (actually i think I'm going to do one of these next!)