A good way to keep assets more cohesive is to decide on a color palette beforehand. Sites such as https://lospec.com/palette-list can be helpful - I find this site useful even though I don't usually do pixel art.
Shadows can make a pretty big difference so deciding on a shading style for all art can also be helpful - soft or hard shadows? Which shadow color? What direction does the light come from? A more technical tip is to use a multiply layer for all shadows with the chosen shadow color.
Being able to adapt to other art styles is a very helpful game artist skill, though that one can take a lot of time to build up. In general discussing what styles and techniques the artists feel comfortable with ahead of time and trying to pick ones everyone feel they can do justice can also be a good approach.
We hope this was of some help and wish y'all good luck in future endeavors!