Do you mean you are using nothing but Choc Malt? If that's the case, i believe you're gonna need a base malt.
Choc Malt is a flavor and color ingredient. It is not as fermentable because the kilning process destroys some of the enzymes that convert starch to sugar.
Base malts include 2 row, 6 row, Maris Otter, etc. they are the malts that ferment best because the enzyme that converts starch to sugar has not been destroyed during the kiln process.
This explains it better than I can: http://www.beeradvocate.com/beer/101/malts/
To answer your question, the yeast you use will impart flavors and characteristics for the style you are aiming for. In other words if you are brewing a Porter there are yeasts that help make it a Porter like WYEAST 1728. A Stout would use a yeast typicle of that style like WYEAST 1332. These are not the only options for these styles. There are many options in yeast each bringing different pros and cons.