I havent noticed it breaking anything. The only thing I can really think of is gardening, where if you shorten seasons then plants have less time to grow and so you'll get less harvest. Still seems to work so you get some though.
I'm pretty sure all seasons stay the same length, whatever you set them too. But when I want an always sunny and warm environment then I play in Sulani or Oasis Springs for a while. It doesnt guarantee warm, dry weather, but it happens more often so they can emjoy the pool :)
The aging process stays the same, so seasons doesnt change that. You might want to change the aging process to fit the calendar though. I use MCCC to have control over specific ages.
Favourite season? It totally depends on my mood! I'm going through a summer phase at the mo, so my sims are living in Sulani and enjoying the outdoor life - beaches, BBQs, lounging by the pool etc. But I think overall I really like the variety of all of them. I like that there's a constant movement, being able to enjoy lounging outside, but then being able to make snowmen and have snowball fights.
I don't shorten my seasons, but I have lengthened them to 14 days each. I found that 7 days was just a bit too quick, and if there were weather anomolies like a stormy/rainy summer, then my sims wouldnt have a nice summer holiday for a whole other year! But with 14 days, I found that I had enough time to settle into the season, and really appreciate the different weathers etc. 7 days I was just focusing on gameplay with a little bit of weather related stuff if I remembered ;)