I've played a few games with seedling swarm now. Some tips I can offer against SM's are:
1: Try to land behind cover
2: Don't try the "too many targets" tactic
3: use genestealers or large squads of gaunts as ferocious to tie up enemy squads early in the game
4: HIVE NODES
5: USE LICTORS!!
6: Without number gaunts are effective but don't invest too many points, they rack up points
There are other good points soon to be discovered but they are not yet revealed.