AdMob may serve you some pay-per-impression ads, but most ads will be pay per click. You can see a breakdown of CPC vs CPM impressions in the AdMob UI. However, there is no way to ask for a specific format.
If you integrate MobFox into AdMob Mediation, and they pay you per impression, then yes, you'll get paid for impression for the ads that fill from MobFox.
Thanks,
Eric