Hi David,
IB made it difficult to trade Forex, and really bumped up the account size minimums. I'm not sure why, but they decided to bail on the retail Forex market. They would be the only ones you'd have to set up contract specs for. Everyone else is just one symbol and you're good.
For Oanda, all the symbols are like EUR_USD. That would be the dollar price of a euro. So if the price was 1.2, that means it costs 1.2 dollars to buy one Euro. Oanda lets you trade whatever size you like, which is really cool, especially for position sizing. So there aren't really contracts. You can buy $23 worth of euros if you wanted to.
Here's an example of how it works...
If the price is 1.12345, and you buy 100 euros, you just spent 100 x 1.12345 = $112.345. If EUR_USD rallies and you sell the position off at 1.20000, you would make 100 x 1.20000 = $120. Profit is $120 - $112.345 = $7.655. You don't get to keep the half cent, but Oanda records it anyway.
From a trading perspective, the interesting part is that Oanda will let you buy on leverage. The example above is leverage of 1-to-1, meaning $1 buys 1 euro. You can go up to 50-to-1. At that amount, that $112.345 would have bought 100 x 50 = 5000 euros. Now your profit is $7.655 x 50 = $382.75.

Obviously, it cuts both ways, so you have to be careful and figure out what the right position size is for you. Margin works the same sort of way as in futures, so you have to have enough in your account to avoid getting a margin call. They also charge finance fees for margin, since you're borrowing money for your leverage, but those fees are pretty reasonable and won't really matter unless you are trading a really long time frame.
There are no commissions. Oanda makes their money on the spread, so you'll always be buying the ask and selling the bid.
Yes, range bars are only for Oanda, as of 2.34 (which is coming soon). That's only for now. I just don't have the manpower to release it in the way that I'd prefer, which is for it to be supported by all the feeds. So Oanda for now, then I'll use that to debug everything and make sure it all works right (I think it does, but I'm always wrong

), and then at that point IQFeed and the others will be added too.
Hope that helps,
Earik