Sorry in advance if this sounds funny or have been asked before. I'm just wondering....

I'm having trouble finding Trace Minerals at the big box store. What brand, what do they look like, etc. etc.?

What are the CONS in just pulverizing a multi-vitamin pill, mixing it with tepid water, and giving it to my containerized citrus trees? The tree will get their iron, manganese, zinc, potassium, vitamin B, among other things.

Can I poison my trees with other minerals that are not needed?

Plants do not use vitamins that are artificially applied. Plants make their own vitamins. However, the minerals (trace minerals ) you mention above might be available, depending in what forum the are (example as sulfates). However, over applying amounts of trace minerals can be toxic to the plant. Further trace minerals have a balanced ratio one to another for proper nutrition. I would not have any idea the ratio nor what the quantitative analysis of the minerals in a vitamin pill would be. Two commonly use trace minerals are labeled STEM (Soluble Trace Element Mix) and Micromax

greenZ,
I usually encountered also problems in trace elements, and my steps lead me to a large nursery. Well, they do not sell citrus, but roses, apples and other berries and garden plants... but the gardeners were able to tell we were to get trace element fertilizer and were to buy a good full solution fertilizer.
So here to most shops have no trace element fertilizers and the most full solution fertilizers do not match the demand of citrus...
So I walked to a agricultural supply store. The sell tractors, harvesters and fertilizer for tank mixtures and have their own tank service.
So I was able to get here all a missed in the shops. Here the personal was quit helpfull and expierienced. Quickly I was able to get my 25 Kilo Bag of full solution fertilizer, 20-5-10 and 2 Mg and all important trace elements.
That's what I found and use, and if I need something special, I walk there and ask... For them, I am only a by-product, a less-worth-income, but I am a well treated customer and that's why I go there....