According to nRF51 Series Compatibility Matrix v2.4 it is NOT OK to use it that way. However, I did use revision 1 or 2 (don't remember now) with SDK 8.0.0 with success. But I was aware of the differences between these chips. Although, I highly don't recommend that for release version of the product :) For development it is possible to do that...
What you need to know is that there has been three revisions of the nRF51 throughout the years. Each revision is an improvement of the previous revision, more specifically, several bugs existing in 1st revision were fixed in the 2nd revision, and bugs still present in 2nd revision were fixed in 3rd revision. Newer SDKs and Softdevices assumes that these bugs are fixed and hence, won't work on old IC revisions as they contain the bugs. However, not all of the features in the newer SDKs and Softdevices relies on these bug fixes and that is why your particular application, using incompatible IC revision, SDK, and Softdevice, might work just fine. What is certain is that Nordic has no way of testing every single use case involving every combination of IC revisions, SDKs, and Softdevices and hence, we will not make any guarantees other than what is stated in the Compatibility matrix, nor will we give any comments on what might or might not work. My sole advice is: stick to the Compatibility matrix.
