I ride with friction shifters with 9s without problems.
I'm not sure if it is the shift gates or what, but I think the more gears, the easier it is to shift. Just about anywhere you put the shifter, it lands in a gear.

I'm not sure I'd ride with downtube shifters and flat MTB bars... (is that what you're putting on?) It just seems like an odd combo. With a road bike, you're already leaning forward, and it is easy to reach the downtube.

If you want simplicity, find some of those clamp-on thumb shifters (very cheap if you buy used).