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In regards to the second concern above, oxygen does encourage acetic acid production in ''Brettanomyces'' (and some species of ''Lactobacillus'' such as ''L. plantarum''). However, a starter can be decanted to remove the starter beer if it smells highly acetic. Some brewers have reported not decanting the starter at all, and the small amount of acetic acid present doesn't make a large impact on the final beer.
If the package is fresh, then a starter is generally not required. If the package is nearing expiration or expired and buying a new package is not an option, or if the batch of beer is much larger than what the package is intended for, then we recommend making a starter for all mixed cultures (including Wyeast Roeselare). In general, we recommend a 500-1000 mL starter (~1.040 OG with DME) for a 5 gallon batch. Run a stir plate on the lowest setting, or use occasional manual stirring with no automatic stirring. Run the starter for 4-6 days. Do not use hops in the starter unless inhibiting the lactic acid bacteria is desired. One may want to have an anaerobic starter in the case that the lactic acid bacteria are especially weak, and the brewer wants to inhibit the yeast growth in the starter. Remember though that yeast needs oxygen in order to grow viable cells.
See the recommendations below from smaller yeast labs below for examples of how to treat mixed culture starters.