Lower ratings of 20C? I sense some confusion.

In an R/C context, a battery with a rating of 20C means it can be discharged at 20x it's Ah rating. So if the battery has a rating of 1 Ah (1000 mAh) you can draw 20 amps out of it. The key thing to know about this is that the battery can only sustain this for 3 minutes until it's dead.

If you discharge a battery at 20C, it's dead in 3 minutes, 10C, 6 minutes. 1C ... one hour.

R/C planes are typically built around 10-15 minute flights, which corresponds to a 4-6C discharge rate. Double that for extreme parts of the flight, and you're at 8-12C, and of course having plenty of extra is nice, which is why 20C or 30C is nice.

But an electric bike should average around 1C or less, so any R/C battery should support a more than adequate discharge rate, as long as it's big enough to last for more than 20 minutes. (Unless you want an electric bike with a tiny battery that only lasts 3 minutes (20C rate), of course.)

9C is a company -- unrelated to the C ratings in batteries.

As for charging multiple 6s batteries, you can charge them separately if you wish -- either add appropriate charging plugs to your custom pack, or add the packs together with a wiring harness of some sort.

As for a good idea or a bad idea, R/C packs tend to give up capacity, low cost and lifetime for high discharge rate and low weight. For an electric bike, you probably want high capacity (more distance), low cost (duh!) and high lifetime (you want it to last many charge/discharge cycles) -- a high discharge rate is likely not important.

But you are right ... even a small R/C battery can put out 35 amps. Like http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=9368 this one - it only weighs four oz. But it won't do it for long unless you get a bunch of them and put them in parallel.