I don't know if peppery is a word but I can tell you that most people will use it anyway. It might be more correct/formal to say "something that tastes like pepper" or something like that, but most of the time people will just take the shortcut and say "peppery". In informal situations, you can get away with putting -y on pretty much anything.
Eldig beskriver bra hur det kan kännas. Svår fråga, men är det inte just chili som gör att det blir starkt/eldigt? I och för sig kan jag tänkta mig att någon som inte tycker om vitlök kan säga att "maten är för stark" om den innehåller mycket vitlök. Känner mig osäker här.
Yes, r and s should be assimilated even over word borders, that happens in most versions of Standard Swedish. There are dialects where it doesn't happen, so it's no big deal if you miss it, but most people pronounce it that way (even if some of those who do don't realize it themselves).
I really don't get this sometimes. Are their rules for för, i, på, om, and all the other prepositions? I don't want to spend forever thinking about it when I'm in a conversation and was wondering if there were specific rules that went along with each one which could help me decide when I'm talking to somebody.
för is not a preposition here, it means 'too' (as in 'too much') and it's an adverb.
för is a preposition in a phrase like en bok för barn 'a book for children'
It's impossible to give simple rules for the prepositions in any language. The Duo idea is to practice typical sentences so that it becomes natural, much like children learn their native language.
You're always welcome to ask about specific sentences in the forums here, and there are already many helpful explanations in some of them.