I have my new minibrew conical in the freezer.
As far as brewing 5 gallons in a 15... I don't see why not as the exposed surface area is the same. However, I have just started the practice of moving to a closed-corny 2ndary fermentation/lagering to free up the conical for more brews.
Also - with the minibrew you don't get a racking arm and it appears to be a lot of beer you leave behind that is below the racking port. I have not measured it but it appears to be about a gallon, so for a 5 gallon brew you will no doubt need to either brew 6-6.5 gallons (which I do anyhow) or you will need to top-off in the 2ndary.