Probably will get worse before it gets better. diatoms tend to burn themselves out after a couple of weeks so dont worry too much about it.
For CUC I would recommend a mix of snails (cerith, turbos and nerites). Avoid hermits if you are planning on a reef tank or keeping snails at all.
As too how many start off with only a few of each. People very often go overboard with CUC which results in half of it dying off because there is not enough food. I would start with a couple of cerith snails after the cycle has finished and then get a couple of each of the others afterwards and slowly build it up.