I mostly go with squads that are around the speed which my 4 Squads have, so for me 450-500.
If you have squads that are to fast, then they might finish the map, before your other squads can kill of pests and will rush trough them.
My set my Squad at around 450 Speed, because it is the normal, considering the range of 200-700, excluding Buffs from certain girls.