It is a good practice not to harvest yeast from beer above 5% ABV.
I belgian breweries they skim the yeast from the surface in the middle of the fermentation, before there is too much alkohol, but I've never tried it...
You could take a small amount from the yeast cake and then grow that up in a 1.040 yeast starter wort. I would not pitch onto the cake or use some portion of the cake to brew another beer. Anything that started much above 1.040 will have stressed the yeast out and they are not healthy and will not do a good job of fermenting your next beer. Another thing to avoid is pitching onto a cake from really hoppy beers as the high amount of hops also stress the yeast out.
I've heard anything above 1.060 stresses the yeast. now this being said if you do a starter than you will be ensured of the yeasts viability and if your pitching rate is high enough you should be able to ward off any off flavors.
I know the pratice is light to dark low alcohol to high. That being said. I pitched the yeast from my Quad (12%) into my Belgian IPA (8%) and a belgian porter (5%) with great results. While i will definitley try to follow the lighter to darker low to high alc. if i'm in a pinch i'll pitch what i have. And i've had great luck. I'de say go for it. Do a starter first.
As an alternative to pitching on the yeast cake, how about washing the yeast and keeping it for later? It sounds like using the harvested yeast with a starter later would be ok b/c of the yeast multiplying in the starter...Thoughts?
starter starter starter. Especially for harvesting yeast and storring it for a while. As long as your starter takes off the yeast is good. (keeping in mind your sterile... no not sterile i meant sanitary. Yea that's it.)