Hi, the characters look really great. I can’t sculpt at all so I’m very jealous of those who can. 
With that in mind I’m going to go out on a limb and offer some advice from a non-sculptor.
How big are they going to be in your game? I guess it’s a FPS? One thing I’d suggest is to work on giving the creatures more large scale details. Even in a FPS often the enemy is only going to be a couple of centimeters high on your screen. Think of it as a painted miniature, the nice surface details you’ve sculpted, like the scales or (maybe?) fur will be lost unless the creature is standing right next to you, and even then, you’ll be too busy shooting at it to notice the beautifully rendered skin.
Here’s a blog post about developing a character from concept to final low poly model.
I chose that one because it’s similar in style to your monsters.
You’ll notice that although the monster has nice scaled skin and high res textures, when you zoom out a bit on the 3d viewer all of that is lost very quickly. What remains visible are the cloth bands around the wrists, the “rib-like” ridges, and other details, such as the clothing and necklace.
If you don’t want to give your monsters clothes, you could consider giving them patchy skin color.
Or some kind of camoflague like stripes or spots.
Or go more colorful, as dinosaurs have become in recent years, since scientists have become convinced that they evolved in to birds
If you want a slime covered alien, either work on adding some ribs, pipes or bulges like the classic geiger alien.
Or just use a good material shader, to get a look similar to the modern “fake autopsy” alien:
To do this I suggest checking out MAtCap textures. These can give a really nicereal-time reflective appearance.
You can use them by adding a texture to a material and then setting the texture mapping mode to “Normal” instead of UV or Object space. They work really well with normal maps, and really bring out the smaller details you have there. (none of those models are mine BTW, just I found them on the internet).
You can use multiple matcap textures on a single model, using a greyscale template image to control which areas will be shaded with which texture. Here’s an old model I did with normal maps combined with greyscale mask and matcap shader. Finally, don’t be afraid to go really low poly with your low poly models. The old lizard man I posted there has less than 1000 triangles, but the normal map really preserves the details.
I don’t want to clutter up your thread with lots of linked pictures, so I’ve hotlinked them in the sentences above.
I really think your models can go from being great to amazing, they just need a little extra something to make them come alive.
EDIT:
I made the usual mistake of not seeing this is a two page post. 
The models on the second page have lots better large scale details. Anyway, carry on with the good work.
