1. My buddies and I brewed a sort-of-weizenbock that used a 4lb can of liquid malt extract- 60/40 mix- and a 3.3lb can of light LME. This beer was (as you would expect) a lot loghter than a weizenbock, and settled much more than a true wheat-beer. However, due to the yeast (WLP300), it still had the classic hefeweizen taste. I'd say go ahead, expect a lighter than usual beer that settles more than a wheat but still tastes great, though not traditional.
2. The above beer was the second batch, on top of the cake from the previous (true) hefeweizen. We even out a third batch (a true dunkelweizen) on the same cake with no problems. In fact, excellent results! Both beers took off much faster than the original hefeweizen. Although the residue on the sides of the carboy looked gross, we didn't have any sanitation problems with any of these beers. If you find a yeast that you like, I'd heartily recommend fermenting a couple batches on the same cake.