1 - I don't remember anyone having trouble since many people seems to use HappyChick. Anyway, you need to know it's a 100% pirate software that uses cracked apps and games, so there's this potentiality. I would recommend to avoid it, but it's up to you.
2 - The legacy rom (Skelton's courtesy) is available to all models. You probably noticed that the legacy rom came full of apps, emulators and other bloatware that you may enjoy it or not. Legacy Rom has a different approach, it's a optional firmware that Skelton has developed that comes as clean as possible and it's highly optimized. There's no HappyChick, although you can download it. This rom is faster and allows you to reach 1.8ghz, since the stock firmware would allow only a maximum of 1.4ghz. Also worth mentioning is that since version 3.4.2 the stock rom has the bad mali drivers (graphics drivers) that won't allow you to play sega saturn using yabause. The legacy rom has the good mali drivers. Surely you will have a lot of fun using the stock firmware, but the best option it's clearly Legacy Rom:http://boards.dingoonity.org/gpd-android-devices/(rom)-legacyrom-kitkat-for-gpd-xd/
3 - Since the legacy rom it's already optimized there won't be much else to do. I just replaced my launcher with Smart Launcher Pro which is the best for a gaming device (in my opinion). You can also use a app to control cpu frequency like nofrills, setcpu or any other depending on what you are intending to play. Also i'm using greenify to hibernate a few apps, but basically that's it.