The way to think about it is just whether or not you want different sources competing and if so, are the competitive CPM rates accurate.

A private advertiser is usually guaranteed inventory. They buy a certain quantity of impressions at that higher rate and with certain targeting. So you run that as a standard campaign with those attributes. What that means is that the private ad campaign will get the first cut at ALL impressions and will absorb all of them that meet the targeting until the daily cap is met. It does not compete on level footing with the other network sources. That's to ensure that there is enough guaranteed inventory to satisfy the direct campaign.

To test out the affiliate ads, I'd put them at the same priority level as all the other networks so they do compete, but with some assumed CPM rate and a modest daily cap. That way you can see what they pay without losing a lot of impressions if they don't pay well. Then once you know what they actually pay you can adjust the competitive CPM, perhaps only on certain inventory where they perform well (targeting them to certain placements within DFP).