Hey David
Interesting question. First is a bit of how you use the word compression, tubes have a very wide dynamic range and, as such, don't compress in the same sense that compressors do (i.e. reduce dynamic range of a signal). However, clipping a tube (as we do to make distortion does make the softer parts of the signal louder comparatively, so from a guitarist perspective they dynamic range is flattened (not that tubes can't be used in a compressor circuit, the better ones do, but not to compress (in a pure audio sense))). So I'm not sure about a compression curve as such. The distortion is caused by clipping, feeding in more than it's designed to take (the design of these tubes comes from making them as clean as possible), fortuitously for us guitar heads, tubes clip giving off even harmonics which sound nice (as we all know and use). So any distortion curve will relate to how a tube reacts to being over driven into clipping.
Re 12AT7 (there are some other posts relating to this model and how it reacts after a 12AX7, and also used as a phase splitter in a tube power amp). 12AT7s are bright (a bit harsh), less gain and higher current than 12AX7. Better choice may be 12AY7, more rounded than 12AT7 but similarly less gain than 12AX7.
Personally, I'm really liking the 2 Mullard 12AX7 long plates in my MP2s.
But let us know how you go with the 12AT7
Cheers R