For the players who doesn’t like to play pvp, getting swarm/locust skins for characters that they can’t use is a waste of time (ToD or event rewards).
So, maybe for the next Gears (I would like this implemented on Gears 5, but there are more important things to add or fix than this idea), you can customize how every swarm or locust appears on your lobbies.
Or maybe TC can do some events including different skins (winter variants for icy maps and/or desert for arid/desert maps).
I doubt this could be problematic for players to differentiate some locust or swarm with a different skin, but if there are skins too similar to a COG player (imagine a skin for a swarm with a broken gear helmet as trophy), that should be banned for customize in horde/escape.
And pve players can use their gained skins (or even buying some)