Is it normal that Quartus has approx 250 warnings when building Minimig? I'm thinking not just for C-One but for all other target systems.
Yes, that's normal. The massive number of warnings does sometimes make it easy to miss the ones that really matter! (If you're seeing a warning about timing requirements not being met, by the way, chances are it's due to the TG68 softcore processor. It runs a maximum of 1 clock in 4, but Quartus is analysing it as though it runs every clock, so it fails timing analysis even though it should be OK at the speed it's actually running at.)
(Yeah, I'm dabbling with things I don't know enough about, but one day I might get there!)
I'm only a beginner myself - I only started tinkering with this stuff at the beginning of this year - but it's the only way to learn.
The hard part is finding a pet project to start with that's both interesting and within reach. For me it was the Pong core I put together for the Chameleon, using the Chameleon's Hardware Test project as a springboard.