The Angara family of rockets is not meant to compete with the SLS and they're entirely different classes of rockets. The SLS could be described as a super-heavy lifter, and the Angara as a heavy lifter.
The most primitive form of the SLS (without any additional boosters) is planned to be able to put 70 metric tons into low earth orbit.
The most beefed-up form of the Angara is planned to be able to put only 23 metric tons into low earth orbit. Its most primitive form puts only 3.8 metric tons up there.
The middle core that looks like the boosters (assuming that's what you mean by second stage) is the same according to wikipedia - http://en.wikipedia.org/wiki/Angara_%28rocket_family%29#Anga... - it's just throttled down after launch so that the external 4 boosters run out of fuel/oxidiser faster.
The SLS probably can't do exactly the same because the solid fuel boosters aren't the same as the core which uses cryogenic liquids. However, if the vehicle can take the stress of 4 solid fuel boosters (they're really powerful) then I suppose it could be done.
There's a good history of scaling to evolve rocket families - e.g. the Saturn I -> Saturn V, and Falcon 1 to Falcon 9/9-Heavy etc. Actually now that I think about it, the odd one out here is the STS/Space Shuttle, which never really could do that with it's funny Orbiter-on-the-side design. Pity.
Incidentally, Atlas V uses the RD-180 engine, which as two chambers. It is derived from the Soviet RD-170 engine with four chambers, the world's most powerful rocket engine. The RD-170 series was used by Buran (as boosters) and in the Zenit rocket (Sea Launch, also Boeing involvement) that still flies.
The RD-190 used in Angara is a one chamber derivative of RD-170.
So you could say that Angara 1 is a "half of Atlas V" or "quarter of Zenit". Angara 5 then would be "2.5 times Atlas V" or "1.25x Zenit".
But the staging improves payload, so it's better than linear.
The Russians have been wanting to do such "universal rockets" since the dawn of the space age. They only managed to build one, so it wasn't much of a modular system: Proton (The UR in UR-500 is from Universal Rocket). The first stage has six engines attached to six oxidizer tanks. But storable propellants were perhaps not such a good idea after all. There were big battles between Chelomei and Korolev about this. Proton has not been used for manned launches.
Angara seems like a very good ideal compromise design. Engines are small enough so it can be modular. It uses nontoxic propellants so it can be used for manned launches. Yet it still puts to good use the fantastic RD-170 engine technology, into which massive investments were made back in the day.
Reusability and competition with SpaceX: Russia has proposed to make the universal boosters of Angara reusable. Add a scissor wing and a jet engine to the nose and you get Baikal. http://www.russianspaceweb.com/baikal.html It seems like a coherent design to me. It will add some mass though, but might be operationally flexible if the boosters can fly long distances. It would be costly to develop, but they have indigenous expertise on these subjects.
Europe hasn't been willing to put money in big liquid engines, so they've needed the solid boosters for Ariane V (which was oversized, since it was designed for the Hermes mini shuttle). Seems they are going further down that road, to all-solids. No hopes of reusability. Or manned flights either. Maybe geopolitical reasons or lack of money.
SLS: it's problematic because it's so big and inflexible. There's no money to fly something so big so often. So it will have high fixed costs. Better than Ares though. None of the solids or engines of SLS will be used for commercial use anywhere else.
Saturn I to Saturn V. There wasn't much commonality, just the S-IVB stage (second in Saturn Ib and third in Saturn V). First stages were completely different, in structure and engines. Saturn I had to be whipped up quickly to test Apollo hardware and train, it was a kludge. There were plans to migrate Saturn Ib to use F-1 engines later, instead of the eight H-1.