Should I setup a new mining pool for litecoin, or join another one ? It seems like creating a pool with p2pool, I can keep fee's in my wallet... But is it effective ? Or staying in a crowded pool will be more profitable ?
It depends on your mining rate. If you think that you have a hight enough mining rate you could create your own pool. And by a high enough mining rate, I am talking about something like 200 Thash/s so you can get Bitcoins fast enough.
If you have, lets say, a mining rate of 1 Ghash/s, it will take your months, even years to mine a complete block.
So if you are alone and do not have a high mining rate, you should join a pool.
P2Pool does not exactly allow you to create a pool, its more like a normal pool than anything else. But I havent tried it so you should still take a look at it. What looks the closest to a real pool (that is in fact not a pool that you will create) is TripleMining. They work with a refference system, which mean that you have to give your personnal link for someone to join your pool. You will get 1% of their profit.
The only downside to TripleMining is that their mining rate is low, it takes them around 200 hours to mine a single block. You will get a lot per blocks, but it will take a lot of time to get your Bitcoins.
EDIT: Someone made me notice that you are talking about Litecoins. The mining work the same way as Bitcoins, just the way calculations are done is different.