Is 8...c5 good in this position, or does it restrict Black's mobility too much? [fen "rn1qk2r/pbp2ppp/1p1bpn2/8/2P5/5NP1/PB1PPPBP/RN1Q1RK1 b kq - 0 8"]
[White "Villain (2075)"]
[Black "Hero (1950)"]

8... c5


I was Black in this position. To me, it immediately looks like the position calls for 8...c5: I need to prevent White from taking over the center, I need more space for my pieces, and I need to protect my queenside from attack on the long diagonal h1-a8. My plan was to play c5 and Qc7 to target the center with all my pieces.

However, after considering the move, I thought that it does not help me much: after I have played for example c5, Qc7, and Nc6, I didn't see any possible continuations. My bishop is going nowhere on the d6-h2 diagonal, and my knights have no good squares nearby if white protects d4, so I would probably need to continue with 0-0, Nfd7 and f6 to put something on e5 to get any control of the game. I also dreamed of building an attack with something like h7-h5-h4 but decided that it takes way too long and white has too many ways to defend.

So I decided that I really need to keep the c5 square free in order to have the choice to play Nc5 or Bc5 (or even Bb4 at some positions) later. 

However, the queenside still is a problem, so I ended up playing 8...c6, which allowed 9.Re1 threatening e2-e4-e5, forcing me to block the e5 square with my pawn, resulting in a loss after 12 moves of misery (apparently 9.e4 would have been even better). 

After the game, my opponent said that I should have played c5, but didn't say anything after I mentioned that I thought it restricts my mobility too much. Was it really a good move? Was it the least bad idea in this position? Is there something better? How can I know the answer during the game without calculating every variation?
I think ...c5 is a good move. The c5 square is not available to your pieces (his d2-d4 would kick your pieces) so you may as well occupy it with the pawn. 

...c6 is horrible because you block your b7 bishop which is currently your best piece! 

Your position is slightly inferior because White has an extra central pawn available (he did well to manage to exchange his b-pawn for your d-pawn, effectively: he now has a half-open b-file to use, and he has better central control).

You can't allow him to play d4 and e4, you will get crushed. ...c5 stops d4 (or at least, means you can exchange that pair of pawns). At this stage I'd be looking to meet e4 with ...e5 , which would shut down both of his bishops (It harms your bishops too, but his bishops are better than yours, so you gain more out of this deal).   

You'd then try to find good squares for your knights. He will undoubtedly go for Nd5 so you'd probably have to exchange that off.

I tried to check this position in opening references, but Black has basically never fianchettoed the QB in response to White's system here. We should conclude from this that it's a bad idea. It seems that your QB is strong here -- although you have to watch out for tactics with his knight moving -- , however your other pieces are all uncoordinated.

The most common way to create and support a centre as Black against White's system is ...c6 and ...d5 . I'm guessing that you played ..b6 and ..dxc4 where it would have been much more solid to play ...c6 which is a standard variation against the Reti. It is slightly passive because your bishop is behind its pawns but this is not a serious problem so long as you develop your other pieces smoothly (similar to a reverse Colle System).