Admob is perfectly okay.

We provide our own in-app billing SDK that supports OpenIAB: http://slideme.org/developers/iap

We only permit in-app billing from Google Play if it is for non-essential virtual currency in a game; that is, it must be completely optional and obtainable in-game also so that users without Google Play are not complaining they cannot purchase it due to lack of premium currency. Any other uses, such as unlocking features, purchasing premium-only items, removing ads, upgrading the app, gambling currency etc., are not permitted through external market billing and will cause application rejection.

There is no point in keeping Google Play in-app billing only when you distribute to other stores since it will not work in those other stores for those devices who are primarily using that marketplace. Greater than half of all Android devices do not support any Google Play services.

However, if you do include in-app billing from Google Play for ONLY virtual currency (such as coins), then we will permit it in those cases. You must set the "In-App" Billing field to "Yes" to be approved, but this application will not be eligible for free SlideME Promotions or featured status due to Google Play billing.