I don't think it's a good idea to play Fed Cup for Mona. I don't think she can handle the pressure there. However all the Germans don't look that good at the moment.
Kerber - Not at her best and probably still injured. She is still the best German though and will play singles in Fed Cup surely.
Barthel - Very unstable mentally and after what happened in her last match I don't think it's wise to pick her to play singles in Fed Cup. I love Mona but that's what I think. I just can't see her handling the pressure there.
Goerges - She isn't really any better than Mona but at least she has some experience in Fed Cup.
Lisicki - Not really a better option than Mona but she was playing decent tennis in Charleston.
Petkovic - Don't think she is gonna play because she is coming back from injury.
So yeah, I really wonder which one of the other 4 Rittner will pick to play singles with Kerber. Picking anyone of these 4 atm seems equally risky.