Diatoms, ammonia and cycling

Hey all, just a bit of suggestions if possible.

I currently have a diatom outbreak starting. I'm nearly at the 4 week stage of cycling and a week ago added a red shrimp, a star fish and 4 turbo snails. What I'm asking is

1)The shrimp just shred and I have got what I could out of what was left in the tank, not sure if this is causing an ammonia spike but it looks like my Ammonia is slightly creeping. My nitrites have droped slightly to 0.5 and nitrates are at 20ppm

2) with the diatoms should I leave them or siphon the sand or get a lil critter to help

3) what % water change shall I do. I was thinking 20%

4) any advice going foward

Thanks all.