Just finished the game for the second time and I still didn't catch the part where their position is clearly specified within the organization.
My first intuition would be that by the end of the game they had already been Lieutenants as they had their own rackets as Tom mentions during the chat with Norman (in one scene you can see him bringing the money him and the boys had to kick up to Salieri) and they were occasionally ordering people around.
So they were definitely not stuck as Soldati but I don't see them being Capo's either as they were still being sent on dangerous missions and had to put in dirty work. In the Mafia 2 loading screen you can see several quotes depicting Tommy's legal confessions and one of them says how him and Paulie (Sam not mentioned) were carrying out risky hits while the bigger guys from the organization were staying hidden in the shadows.
This is also why I have the impression that after Frank and Morello's deaths Sam was a rank or two higher than Tom and Paulie as he does the best out of them within the Family (secretly plans with Salieri to steal dope, has his own nightclub and in the final mission at the Art Gallery he commands an entire squad of goons), so he might've actually been a Capo.