AS16 and ASCA are designed to work together. ASCA provides the textures appropriate to the season, lighting, geography and intensity of the weather (calm, stormy, overcast, etc.)
Without AS16, only the static themes are available, the dynamic themes are not. The static themes however can easily be underestimated. If you have a certain look you expect of the clouds no matter where and when you fly, you can try out the various static themes, find one to your liking and use it all the time. No different than loading a cloud texture set before you fly.
You still get lots of variety even with a static theme as you do with all FSX cloud sets because of the seasons, lighting, conditions, etc. CBs with TS, different kinds of cirrus, mix of layers, etc.
One of the most outstanding features not discussed much here is something the other texture tools do not do. Load textures on the fly- AS YOU FLY while FSX is running.
Among all the static themes from which the dynamic themes draw from is over 150 different cloud textures and cloud types usually not contained with other texture libraries.
With AS16 and ASCA working together, ASCA will dynamically choose appropriately from the different theme library sets ON THE FLY. This adds thousands of variations to your flying experience you will never get with static libraries.
With that said, the simple way of thinking is you can choose a static library which has the type of cloud appearance you prefer to fly with all the time. This does not mean the clouds will always look the same. Or, you can (with AS16 running) let AS16 dynamically control ASCA's theme switching to provide infinite variety. You may not like every cloudscape generated. We don't always flip over the weather outside our home, but since the dynamic themes are also able to draw from your favorite library (among others) there is the chance you will see portions of your favorite theme once in a while.
A "Static Theme" is a combination of various low, mid, and high altitude non-cirrus, a collection of cirrus, and lighting intensity and coloration.
A "Dynamic Theme" is controlled by AS16 and is a mix and match of Cloud, Cirrus, and lighting from each of the static themes, not always from the same theme.
Since there is no better weather engine than AS16 and there has never been a better weather engine by anyone other than HiFi's Active Sky, there is no reason and no advantage to mixing and matching another weather engine to ASCA's effects. On the contrary, you throw away many of the advantages ASCA provides.