Hi Gregory and welcome to the Guild!
I don't use CC3, but I believe that their symbols are in .png form, although how accessible they are for mass exporting, I don't know, I suspect that Profantasy would frown on the process for fear of their symbols being lifted wholesale and might make it difficult, which they would have every right to.
I've always found it challenging to try to get something consistent by mixing up different symbol styles and map elements, sometimes it hangs together but mostly it doesn't, unless you start modelling the look of your map on the look of your symbols. That said, because you've kept the land textures reasonably neutral, I don't think your map suffers too much from the symbols clashing with the land textures. The fact that the symbols are oversized helps as well.
The reason why I think you may feel that it looks like a mess of clashing fonts and symbols etc is that you've tried to do too much with it. For example, to my eye, the circular LOTR image insets are a great idea, but I don't think they fit. You could try making a border with circles in the corners and putting the insets in there appropriately labelled. Suggestions of things to try:
1. Lose the background 'knobbly' texture, it doesn't add to the knowledge of the topography and IMO is distracting. Use a more subtle texture if you want one, maybe vary the texture between the sea and the land as well.
2. The coniferous forest looks like it's been cut out and dropped in (the edges are very smooth), do a search on this site for 'the tree thing' which is a java application created by a couple of members of the guild and which will generate individual trees ordered on a z axis to a mask pattern you create. It's more complicated to explain than just to see, so have a look at the thread to see what it can do. It will give you a much nicer fir forest and one that looks like it's been drawn with individual tree symbols.
3. The scale of the dead tree symbol is huge compared to the coniferous forest symbols. Maybe make them the same size (there may be 'in game' reasons why you want them different, but it looks odd to me). Again, the Tree thing might help you here.
4. Consider getting rid of the drop shadow effect on all the CC3 symbols as it makes the elements look like they are floating.
5. In terms of colours, I find that keeping a roughly consistent colour saturation level (darker colours slightly more saturated and lighter ones slightly less saturated) really helps with consistency. The coniferous forest green and the symbols are supersaturated against the rest of the map colours. So maybe desaturate the symbols or increase the saturation of topography.
6. If you are using photoshop, why not use the perspective tool to tilt the whole topography a bit into perspective, it might look good with the iso symbols?
I'll knock up a quick example of what I mean, but that's as much as I can suggest for now.
best and good to have you here,
p.s. we would love to see some of your GIS work if you're willing to share!