5g - should be fine addding shrimp or snail, ghost shrimps may become snacks for the betta. Might want to try a bamboo shrimp since they're a little larger.
10g - a small school of neons, glo-lights, cardinals, or rummynoses would be good for a schooling fish. I prefer rummynoses since they tend to be more active and school better. I also like panda cories for bottom dwellers. 6 rummynoses and 4-6 pandas would be ideal.
35g - Another 4-6 tiger barbs should be fine.
75g - 1. Sand isn't too difficult to maintain. You have to cover up the intake to your filter with a pre-filter sponge or mesh, to prevent sand particles from damaging your impeller. You don't want to have too deep a sandbed unless you have rooting plants, anything deeper than 2 inches can be dangerous because sand can form anaerobic bacteria pockets in areas where it becomes compact. Anaerobic bacteria release methane gas, which is toxic. Stirring up the sand occasionally will also prevent this.
2. Think you want 2-3 females per male. Don't get more than 3 males since they're territorial against other males.
3. Larger schoolers like congo tetras, blackskirt tetras and rainbowfishes. For non-schoolers, angelfishes should be compatible.
Easy to care for plants in a low light setup are:
anacharis
java fern & moss
crypt wendtii
anubias (any)
hornwort
Easy plants that will survive in low light but do better with more light are:
water sprite
water wisteria
temple plant (hygrophila polysperma)
amazon sword
dwarf sag
vallisneria
. You might try a brig snail or 2. They come in tons of pretty colors. Some bettas get along with snails, some dont. I have found that most of my bettas have done well with other fish, but not guppies. Betta's tend to attack them bc of the flowing fins.
