Yeah, the PC2700 RAM will just get clocked down to the speed of the slowest module you have installed so it will all run at PC2100. Most likely it will work perfectly, but RAM can be fickle. Sometimes two sticks won't work together even if they are the same speed, but just different manufactuerers. And sometimes your system won't boot when you plug your RAM into slots 1 and 2, but if you use 1 and 3 it works fine. For $17 I'd give it a shot -- especially if you only have to send in a copy of your receipt for the rebate. Then, even if it doesn't work you can just take it back and you'll still get the rebates for your trouble.
If both sticks of RAM have the same number of pins I'd give you a 90% chance that it will work just fine. You probably have just as much chance of it working as another brand of PC2100 RAM. That's just the way RAM works.