As to the first paragraph, over the past 10 years, stem clamp diameters (where they clamp the bars) come in 3 varieties. 25.4 mm, 26mm and 31.8mm. Plenty of drop and flat bar choices in 31.8. plenty of drop bar choices in 26. Plenty of flat bar choices in 25.4 and maybe 4 or 5 drop bars in that clamp diameter.

Oops, didn't look to see what forum this is in. No need to worry about the STI action.

Is the mountain bike you already have a singlespeed at the moment? If not, I wouldn't bother with the conversion. Just wait 'til your 70 pound bike gets in. Man, that is cheap. I'd be almost scared to ride it