The short answer is if you're just starting out with a new garden you shouldn't have to worry about using too much compost. If it's mostly leaves from trees you shouldn't have to worry much about toxicity. Incorporate lots of that County compost and get your soil tested at least every other year. Maintain soil organic content and structure by mulching and not tilling more than necessary and you'll be rewarded. Adjust pH gradually and as needed to 6.5 to 7. Uncomposted organic matter that is low in nitrogen can deplete N during the breakdown process, so nitrogen fertilizers may be needed when plants are growing rapidly, especially in rainy seasons, as nitrogen is susceptible to leaching.
Compost derived from manure from animal feedlots or from crop residuals that have had lots of herbicides applied to them might be risky. Long term application of composted manures can boost soil phosphorus levels above optimum. The adverse consequence of too much phosphorus in your soil can be a loss of connection between plants and mycorrhiza. And no, mycorhiza is not something you buy in a store and apply. It exists in healthy soils with plenty of organic material. When you lose that connection you are essentially growing in a system where most of the measurable nutrients might not be plant available.
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