My current stumbling block would be devising a small group for domo. Rag could, arguably, be considered easier than him with a small raid size, though 20 is a trifle low for maintaining stamina through multiple sons phases.
Domo would be very hard. 6 Tanks minimum (4 for Elite/Healers, 2 for Domo), 4 Mages minimum (4 Sheepers for Elites/Healers). Thats 10 and you haven't even started to address healing and DPS. Trouble with MD fight is is really does have to be fast, else you just run out of mana/hps. With the magic/melee shields halving your DPS for 10 secs in every 30 you need more than the normal DPS to take out MDs guards.
Reckon it's doable with 30 if your pretty heavy MC equipped (we've cruised it with 35 but losing 5 people can make quite a difference), if your BWL equipped you're looking at less, you might get it down to 20ish then, though 5 healers and 5 DPS sounds very, very light to me.
Ragnaros is more interesting, though by no means is he an easier fight than Domo. Rag is tactics 50%/ DPS 50%, but even with the best tactics in the world if you're facing more than 2 or at most 3 spawns of Sons I'd consider you dead.
You could certainly sacrifice Ranged Magic DPS on Rag. basically dropping your Ranged
contribution to Hunters only and relying on your Rogues/ DPS Warriors (Salvation Blessed if ya Ally) to carry the day.
Even so if you take the heavy melee/low magic option then your melee FR has to be through the roof, cos with reduced healing power (small raid, something else has to give too to keep the DPS up) your healers are going to be very taxed mana wise - even assuming the Sons tactics are spot on and they're not losing much to mana burn.
Again for small raid Ragnaros, you'll be looking at full MC equipped minimum, heavy emphasis on FR gear (high end Thorium Bro. made stuff + MC core farming alot), the Fire damage output on Rag is brutal to say the least.
Onyxia actually gets easier with less, she's easily a sub 20 doable raid-boss if your heavily enough DPS gear loaded, I actually find her harder atm with 40 than if we do her with 30, simply cos of phase 2 spreading to avoid Deepbreaths. Atm I'd guess my guild could kill her with sub 30, though not less than 25, with BWL gear it would be a breeze though.