Utterly Mad

The bad news is that sebillians are by far the easiest thing you're going to fight. The good news is that this will become more apparent as you gun up a bit.

If you're on easy, and pile on the gas (it does compound the effects to a degree - you can see this visually), you should reliably knock them out in a single turn. This goes for anything without a helmet you'll see early on. This works less well on normal and above.

There's a couple mods that make it so your foxtrots can do ok vs. fighters if you're manually controlling them (the one I'm thinking of lets you swap the torps for missiles), but otherwise, Condors all the way. They should be able to 1 vs 1 them ok until heavy fighters show up. If we're talking the smallish 2000 speed scouts (not actual fighters) the the reverse should be true. It's been a while since I played below veteran, but IIRC keeping the skys clear isn't too bad for the first couples months since you can take most things down with only two planes.

I usually just skip the corsair, TBH, because the right mix of foxtrots and condors should be able to take on anything even into the later game. Once you get the marauders, and enough of them, you start to feel kinda bad for the aliens.

Other bit of advice I'd maybe have is ignore everything except bravery and reaction when recruiting. The rest of the skills are very easy to train up, but bravery is hard (if you aren't feeling gamey, anyway), and I don't know of a reliable way to level reaction.