There is a way to equip any faction armor on any follower.

1) You'll need 6 or more of the SAME armor and they have to be in the SAME condition. For example, if you want to equip Desert Ranger combat armor on Cass, you'll need 6 or more Desert Ranger combat armors, then take them to a vendor and have them all repaired so they'll be in the same condition.

2) Make sure the armor you want to equip is stacked in your inventory. Example: Desert Ranger combat armor (6). Open your followers inventory and select the armor in question. You should then have a slider indicating how many you wish to give to your follower select 1 and you'll notice that this way the armor remains in his/her inventory and will be equipped if its the best armor they have to choose from.

NOTES: The reason you'll need 6 or more is because at 5 or lower you do not have the slider to choose how many you wish to hand over to your follower therefore you hand them over one by one, doing so will make them "drop" the armor and they won't equip it any other way.

After going to a vendor and paying for repairs the armors might still appear individually in your inventory, if this happens you'll need to find the appropriate armors to repair them with to 100% condition, as you do this they will stack up in your inventory. Also, if you already have a healthy supply of armors you can save some caps by making the repairs yourself.

Remember that when you'll take away the armor from your follower it WON'T stack up with the rest. The same applies if you have 5 or fewer of the same armor and you put them in storage, since you won't have the slider to indicate how many you wanna drop you'll store them one by one and they will not be able to stack anymore. Unless of course you wear them and damage them, then repair them all again.