You might want to control the way some polygons fold with animation by manually triangulate. It's more an issue on very low poly tho. For the engine, it's the same.
The number of objects may be a bigger problem than texture size or polygon count. Have you read the optimization guide? https://udn.epicgames.com/Three/LevelOptimization.html
that did it! I also found another way to do it in default rendering: Shading --> Polygon Transparency Sorting Thanks guys! This makes it way easier to model.
You split the individual toes while retopologizing, by building those in-between polygons yourself. Alternatively, it is possible to trim the gaps in there, but that would be messy and more difficult.
Very nice environments , but are those in game or in 3d render? in case is the first what engine is that? can we have more info on polygons , textures etc?