Do the changes I make to the default races, effect all of the NPC's that use that race? I want to change the skills/attributes of one of the races. But I don't want to break every NPC using that race. I intend to change the Khajit around a bit to make him more of an Unarmored/HtH fighter. With some supportive magics, and attributes to support that. So he will have very little stealth skill.

However, if there is already such a race like this, id be happy to know. I looked through some races on telephoros list. But pretty much none of them tell you the attributes of the new races, you would have to actually download them all and look in PES to see.

I don't think so. if you change the basic starting attributes of a race you should be fine. Other NPCs have fixed skills/ attributes assigned to them in most instances - the autogen check for leveling is quite rare in morrowind. I think I can say as a certainty that whether Khajiit can sneak or not will not break any quests in game, because the results of any scripted behaviour are fixed.

Changes to the default races will affect all NPCs of that race and gender (you have to choose which gender to change), but as Brakes Brakes Brakes said, those changes shouldn't affect NPCs because most of them use auto-calc skills and I've never seen a vanilla NPC sneak other than Fargoth during a particular quest. If you do change the h2h stats of the Khajiit, you might have more cat bandits punching you, but that'll be about it.