I'm not sure I understand your drain question. Is this for water going to the sump from the main pool, or is it a cleaning drain?
If the former, you will need the openings to be at the planned water surface, so you will need a standpipe of some kind. Will you have a hole in the bottom of the tank? If so, fit it with an appropriate bulkhead and use a length of potable-grade PVC pipe slightly taller than your pool's intended depth. You can use black pipe for lower visibility. Drill several holes where the waterline will be. Insert the standpipe in the bulkhead and stuff it with filter sponge. Conceal with driftwood or plants if you like.
Appropriate fish include smaller livebearers - guppies or Endlers - or killifish - Lucania or Heterandria species or some of the smaller Fundulus, for example. You could also try pygmy sunfish. All of these fish would appreciate some plants, especially the pygmy sunnies. Appropriate shrimp include ghost shrimp (Palaemonetes) or cherries and kin (Neocaridina).