(Shannon is a long time customer of mine :). Oh, and no, that's NOT why her question got picked. She got picked because it happens to be the email my mouse clicked on today - my mouse has a mind of her own LOL). 

OK, let's roll up our sleeves and tackle your (ever so most excellent question.

When should fry start eating other foods than their baby food (microworms/baby brine shrimp/vinegar eel etc)? As soon as they are large enough to. So here we are not looking so much at a time factor, but more at a size factor. As some spawns will grow much slower than others, putting a time frame would mean very little and result in possible disaster. What you need to assess is how large the mouth of your fry is becoming. If you can fit a bus in it now, then by all means do start feeding busses to your fry (although it might become expensive LOL). A few tips:

• never switch foods abruptly. Instead you will gradually start introducing new foods while still feeding the main fry foods.
  
• always test the waters first (if I may say LOL). So to make sure you have properly assessed their mouth cavity ability! You don't want your little ones to choke on too large a morsel now! Cause you KNOW they WILL try to swallow anything, even if much too large. A piece of advice: Don't drop your cat in the tank! As a matter of fact when I place chopped food in a fry tank (oops, that did sound bad... I was not making any reference to cats right now LOL) (I love cats, too. Let's exchange recipes?? LOL) (oh I am just teasing you guys...), I notice that the runts will not pick bites their size. They are just as ambitious as the big fry and will go for HUGE chunks of food the size of their body (which they cannot swallow) and will not go for the smaller chunks (which they could swallow). In the process I think they often miss the boat and end up with no food at all, because a larger fry will finally get a hold of that huge piece of bloodworm they have been logging around for a while now (unable to swallow it), and will rip it right off their mouths and eat it [note: this sentence has just ended - surprised? LOL]. So the runts get NOTHING. Morality of this story? Don't chew on more than you can bite... wait, no: Don't bite on more than you can chew! No wait.. Don't chew what you bite in? Or is it: Don't bite your tongue when you chew? I can't remember. At any rate, a good piece of advice (that I should really follow LOL).
  
• always chop up the new food as much as possible. Although not ALL foods can be chopped, most can be and I certainly ALWAYS do chop the food I am introducing to my fry. I use scissors to do this (yes). I dip the scissor inside the cup holding the defrosted food and chopchop chopchop as long as it takes until it is reduced to mush hehehehehe... 
  
• don't overfeed. Live food will not pollute the tank if you over feed a bit because the fry will eventually get to it later and eat it. But non-live food (frozen, freeze dried, etc...) will rot after a while (maybe as soon as 1 hour) and will cause a massive mess. Also chopped food is messy by nature. So better go easy on the quantities you dump in there kiddo! Feed more often, and feed less. 

Here is a list of some of the suitable foods for fry growing: freeze dried bloodworms (easy to crush into a fine powder with your 2 fingers), freeze dried brine shrimp (not as easy to crush but still great, will float but can be made to sink by sticking your fingers in the water and then releasing the food), frozen brine shrimp (chopped), frozen bloodworms (chopped finely), frozen mysis shrimp (San Fransisco Bay brand only as other brand's mysis shrimp are too large for bettas :( and would require MASSIVE chopping - SFBB has small size, good for betta's mouth and it will not require much more chopping than brine shrimp).

Notice I do not list pellets because  most are just not nutritious enough to be good fry foods. There are a few exception, but more on this in another column. Flakes? Pfffffeeettt! Forget it!! Live foods? I stay away! My new moto is: No live food, betta is good. Not that live food is evil, but it presents the very strong possibility to be the vector of many a nasty bug that will latch on to your betta's jugulars (matter of speak). So I pass, thank you but no thank you.

Well kiddo, I hope this helped you and 'bon appetit' (spelled the French way ;)  ) to all your little ones!