I want to get Tristian to my party, because I want more of a spellcasting healer. But I did the quest and he is simply stuck in a bed on saying that he's going to get better soon. Did I screw up? Because I have done some other tasks, including finding tartuccio and he's still not available. Does he become available later or do I have do get back to an old save to fix it?

I got him by rescuing him and healing him. Then when I was about to go away he said, "wait..." and offered to join.

He should become available as soon as you're ruler.

He's available before that. I haven't even found Tartuccio and killed him yet, and I have Tristian in my party.

Ditto Here ..Rescue and go through Dialogue.. and he volunteers...

In my case, the characters level up all the time. When i get them, they have the same level as my main character.
I thihk, there is some button in the options. This way, you avoid having underleveled characters...

My game probably glitched, because he's the only one that didnt get the exp, Aetheal and Regongar (the 2 other companions i'm not using at the moment) have received the exp accordingly, so all the companions except for Tristian have enough experience to move to level 5. Well, it happens.... I guess... better start strategizing with 4 on the front and just 2 in the back xD