Even on a small screen, you can setup a great not cluttered UI, it's all about prioritizing what you "NEED" on screen and what you "WANT".
For a small screen focus on the basics.
1. What is my focus in a group or raid..if DPS go to Step 3 else goto 2.
2. For Healers especially in raid setting 2 things are your goal, keep yourself the Tank alive First, i.e...Dead tank = dead group. Dead healer = Dead Tank = Dead group, so focus on mods that help in these areas, Clique, Grid(or comparable raid frame setup), and a HoT timer for certain healers.
3. For DPS..threat is your only real factor to worry about. So get Omen or a comparable Threat meter, if you draw aggro...u die. Quartz is good for casters, and a DoT timer are basic necessities. There is no real need for Grid or anything like that unless you just wanna keep an eye on the raid/party group. But in reality keeps your eyes on the mobs.
4. Beyond those basics all else is glitz and glamour. DPS Meters are a distraction, I use one but it only records boss fights and only shows up after its over. Everything else on my screen is just for informational purposes not in combat or questing.
5. My chat logs are set up for no distractions. The left hand side looks like the standard chat log but it's so much more. There are 3 tabs there, General, City, Raid tabs. They function as that General shows guild, party, and whispers, City tab only show chat in towns, i.e LFG, Trade, city channels. Raid tab is just that...raid related info and whispers only. So if I'm in a raid grp and dont see what you say in guild chat thats why.
6. My combat log is simple yet complex, it is broken down in sections;
Hurt - Any damage I do to mobs or targets.
Self - Any damage I receive
Heal - Healing done to or by me.
Nothing - duh..shows nothing
Group - Shows damage to the group.
Seems complex - Hurt for when I'm dpsing, Self when I tank, heal when I'm healing, nothing when i'm questing, Group is for after the dust settles so I can see what happened to my group.
I have a multitude of mods running but some mods are just overkill, a majority of stuff can be done in-game w/o a mod if you just take the time to invest in your UI.
If you are dead-set against mods, you are shorting yourself and your group out on what you could be doing...but most importantly avoid clutter, redundancy, and distractions, your UI will thank you and so will your group.