I'm not too sure but that you might have a flower stalk unfurling in the plant on the left in the photo where your measuring the height. It could be a leaf, but that really curved-looking stalk makes me think flower.
They do look like young adults, though nice sizes. I have some local plants that I don't think are that large...and they've been blooming for a few months now. First it was one plant, then other plants started sending flags up.
I don't think your current food will hurt the plants. I think lots of people use flakes of different kinds. The sure bets seem to be the FDBW and Betta pellets.
One thing I want to point out is that some people are allergic to FDBW. I think even Matt, that runs FTC and the FlytrapStore, is allergic to them as are other people. Just thought I'd mention that so you can be kinda careful when using it...try not to touch your face or inhale any powder that might be present while feeding it. The Betta pellets (I think the smaller ones are usually recommended) seem to pretty much be foolproof. Just take care not to drop as little food on the soil service as possible.
The plants are definitely big enough to enjoy (and probably need) some nice meals.