Hi Joanna
>> Can I use the soil in my beds if I mix it with compost and booster?
I do. And pre-mixing it with compost is a great move. I figure the extra loft helps my clay soil a lot, and adds organic matter. Any left-over fertilizer will first be diluted by mixing with soil, or by being spread as a thin top-dress layer, and then flushed away by our darn constant drizzle.
If you think the plant in the pot might have had a disease, you might discard that soil, or put "sick" potting soil from flowers into a veggie bed, and potting-soil-from-veggies-in-pots into a flowerbed. That way, any plant diseases will be denied hosts.
Another way to "sweeten" a mix that was in pots would be to put it into an active compost heap. By the time the compost organisms have finished eating it (and each other), it will become compost and many plant pathogens will have been mostly consumed.
