I successfully blended NHL based plaster and gypsum with no worries, don't have a picture though sadly. In one of the cases, had one room where only damp issues were window reveals. Hacked those off, installed new windows, and then re-plastered reveals in NHL lime, used a fine sand for top/skim coat, and could not see a difference.
In other rooms to save some cash I only hacked off and replastered the exterior walls in lime, left internals gypsum. One that one I used a "fat lime" plaster, again a fine sand for my top coat, blended really nicely, no dramas.
Good range of top coats from places like this for example, if you can't be bothered with all the sand sieving: https://www.lime.org.uk/lime-top-coat-finish-plaster.html
Fine sand and lime putty is my preference for the last two coats (at least).
Not sure why putty, I think it is just a mental thing as putty seems almost like a finish coat just as pure putty.
I find the tricky bit is getting the thickness of the sub layers in the lime correct to match with the surface of the original.
I using lime makes this more difficult because of the 4 or 5 layers used.
There's also the option of using a chalk/lime mix with no sand at all. This can achieve a very fine finish.
I'm using this ready-mix with added fibre: http://www.anglialime.com/products/wet-mix-plasters-angliamix-mortars-renders
It's also available without fibres as a topcoat or skim coat.
Lime/chalk ready-mixed filler as well: http://www.anglialime.com/products/wet-readymixes-mortars-renders-plasters/superfine-filler
Have done it on flat walls with NHL based plaster, didn't see any cracks in the 3 or four years since I did it. You can get lime putty based fillers which do a great job if you do get small shrinkage/movement cracks