Lo-Fi Games

To which clan I can join if i will build my base on the swamps? To all of them or it is just The Hounds?

there are at least 3 major factions in the swamps. In towns it the hounds, but outside that it's the red sabers and swamp ninja you need to watch out for. There are several other factions in the towns but they have bases in other smaller towns instead of Shark. You would have to travel to those towns and if you did join with them, I'm sure the hounds would have something to say about it.

But still - I will be able to join any of them?

No, as they are hostile clans and will even attack you if you have "neutral" relations to them.

Only the Hounds can be made allies in the swamp.
The Flotsam will also help you out if your base is in the swamp and you are allies.