Is it possible to change the symbology of a WMS from external developers?
I am harvesting WMS and the symbology is very messy and unclear when some are viewed together.
Not always. Changes can be limited for adjusting the "symbology" because a basic WMS provides a pre-styled map.
There are sometimes controls/options you can use in the GetMap request to control transparency and such - but nothing to actually change the style (outline/line thickness, fill and outline colour, etc...) of the layers as those are built on the server side.
Sometimes you can also add each WMS layer individually. This helps because you can re-order them to draw how you like.
That's the one major restriction of a basic WMS - You will forever be at the mercy of the cartographer. A WFS on the other hand lets you access the raw vector data for your own styling purposes but is not always an option.
There are ways to send an SLD file with the WMS request to ask the server to style the map in a way that suits you. However not all servers (or clients) support this as it is much less efficient (it prevents the server caching the result for other users for example).
See these examples: