Is using XMR-Stak okay here? (sorry for long intro)

I'm sorry if this is an inappropriate place to ask, but I've only just started dipping my toe in the (Monero) mining pool. Pun intended.

I followed a simple guide to getting started with XMR-Stak for another pool (US XMR Pool), and everything was going fine until that pool started having issues yesterday. So I temporarily switched to another pool for the time being to see how that was, and it was fine. This morning I saw a notice on the first pool's website that it was shutting down and some of the people were coming over here, and figured it'd be much better for me to join this pool seeing as I'm in Sweden and the pool is located in Germany, rather than keep mining towards pools in the US.

Now, to my questions:

1. I'm comfortable with my current miner XMR-Stak, but the website recommends downloading XMRig and ccminer. I've given ccminer a try, but I seem to get more juice out of my old 970 with XMR-Stak. Windows yells loudly at me when I try to download XMRig because of triggered trojan warnings. Can I just keep using XMR-Stak?

2. I've been mining for a few days on a couple of different pools, all without any issues. I've only mined here for a short while and gotten a few "block expired" errors. I have no idea what that means, but I very much dislike not seeing 100% success rate on my results page. Is it my choice of miner software causing an issue in this specific pool?

3. The website lists different ports to use for CPU/GPU mining, but my miner uses both methods. Should I choose any specific port rather than the other?

I hope my post is clear enough to understand, and I that I've not taken up too much of your time.