I have found that I have more asteroids than asteroid miner ships so was looking for quality of asteroid group
If not there then please add it for it adds to decision making.
There is only one type of asteroid. they come in various size groups they all give the exact same bonus to the core they are assigned just the larger the field the more each miner collects.
You do not need a miner for each asteroid just one for each field and once you research the miner tech you can build all you need.