Seed shortage tends to sort itself out over time, but brewing the produce promptly ensures seeds are freed up for planting.
A mixed squad is not a good idea, generally. Personally I find marksdwarves too bugged, ineffective, and obnoxious to be worth the effort, but others on the forum disagree.
Wealth is basically irrelevant since the economy is broken.
Getting water into the fortress should be done with care, because any hole can be used by nasties to enter. Drawbridges are the standard answer to that, though.
Getting water to flow isn't that hard: Dig a channel or tunnel up to, but not through to the water, ensure you have drawbridges to control the water flow as well as access to the fortress. Make sure you have the water destination figured out (typically a cistern with a depth of at least two tiles). Note that water flows just fine through staircases, so the tunnel you dig can use staircases for vertical flow. When you've set all the infrastructure up, and tested that you actually build raising rather than retracting drawbridges, you channel the last tile to the water from above, and the water will start flowing. Note that a mega channel or basin will have difficulties filling up with water due to evaporation, so avoid a 50 * 50 swimming pool (you can get around the evaporation issue by dividing the pool into sections so you fill one then channel away the wall, fill up the combined one, etc).
You can do some test channeling up on the surface by e.g. first channeling out two tiles one tile away from a body of water, and then channel away the tile separating the areas.