I met the Gastronorm system tonight, by accident, and discovered it was based on a standard container size of 530×325mm. Wholly logically, other sizes are fractions or multiples of that 1/1 standard, so they play no further part here. I've found any number of informative sites that repeat what I've said, and sales sites offering items matching that system but still, where does 530×325mm come from? If this has nothing to with any culinary art, anyone reliably saying that will be helpful in ruling out various negatives. Until then I just about see that the ratio 530/325 = 1.63 has classical echoes but still, why 530 instead of 500? Why 325 instead of 300? I first thought those numbers might somehow resolve into something obvious but multiplying them gives 172,250mm sq… no more meaningful than the originals. Perhaps they were left over from Imperial equivalents? Uh… hardly. 530mm equates to 20.808 and 325mm to 12.795 inches… no more meaningful. Similar but different, the "A" sizes for paper can claim a historical basis… it was ever thus, for whatever reason… The A-ratio comes from a mixture of maths and philosophy in ancient Greek; the specific sizes in the A-series from simple convenience. By contrast, Gastronorm was apparently invented in Switzerland in 1964 and in 1993 became what is often referred to as Euro standard EN 631. Fine but my Chrome and Google, for one, aren't able to relate EN 631 to gastronomy, except - and not always - by reference to materials used in construction of utensils designed for handling food. Nothing to do with sizes… Going back, the A-series sizes are at least recognised, if not used pretty-much the entire world over but A-series 297/210 bares no relationship to 530/325. 530/325 does give 1.63, sometimes called the Golden Ratio and seen as philosophically, if not mathematically important but then why not start from, say, 500 or 300… why 530/325? 
The key concept is the food safety 'danger zone'. There is no reason to treat bones (which will have traces of meat on them) differently to meat. Whatever temperature they start at, you want to avoid the bones spending too much time in the danger zone temperatures. Starting from fridge temperatures rather than freezer temperatures helps this because your stove or cooker will be able to bring them to a cooking temperature faster. As long as the quantity of bones isn't huge, starting from frozen in a pressure cooker or on a stovetop should have no problem getting to a cooking temperature quickly, but a slow cooker on its low setting might be an issue, as you have noted – follow the advice for frozen meat of using the high setting initially. 