to start tuberous begonias in very small pots or all on small flat bed of soil, and then transplant them later at planting time (mid-May) into their final pots?

Or to start them in the pot they are destined for? These are all to be in pots, not in the ground.

About half already have very short sprouts (I picked through lots of bags to find ones that did!) Should I handle planting differently for the ones that do vs. the ones that don't?

And: my house tends to be cool, only about 65 degrees F. Do they need heat to sprout? And how do you provide it? I have no grow light but could maybe use an incandescent clip light or two and a timer: but that would mean starting them in the basement, where there is little natural light.

I have 12, mixed colors. Once I know what color each one will be I plan to add very small accent annuals to the pot, like lobelia.

Choose pots which are not too small, though. In fact, try to get pots which will allow atleast 1" of space between the edge and the tuber. This will ensure good rooting.

The pots should have 1.5-2" of soil in the bottom. Plant sprout side up and cover with 1/2 to 1" of soil. Then water well once. Use a well draining medium, such as seed starting soil, or try potting soil mixed 50% / 50% with perlite (you can find these white puffy things in 4 qrt bags at the garden center).

If you do not have flourecent lights, then start them in a window which is facing east or north. Turn pots frequently to avoid plants growing towards the light. I used to use an eastern facing window with a white sheer curtains to block the light in the morning. In the afternoon I removed the sheers for better light.

When transplanting, do so as the plant is 4-6" tall and can hold the root ball together.

65 deg F is just fine.

Tubers without sprouts, you can plant as usual, but do not cover. leave them in the pot and as soon as they sprout, cover with 1 - 1.5" of soil.

Try this link for a picture-by-picture guide for how to start tubers: Guide

Here are some pictures: