Advice on feeding 2 kittens recovered from inside kitchen wall today

Last night, we heard some noises coming from inside the wall in our kitchen.
This morning, it became evident that these noises were at least one, and
most likely two, small cats.

Three squares of plaster and lathe removal later ... missed on the first
attempt, plus we have an old house so it was a bit tougher than cutting
holes in sheet rock ... we had pulled two kittens out of the wall, one a
cream colored tabby male, the other a smoky dark gray to black female.

Obviously, a mama cat had crawled underneath the house and given birth, and
somehow ... and we still don't know exactly how ... these kittens ended up
inside our wall.

We are a cat family ... although our current cat, a nearly 4-year-old Maine
Coon/Norwegian Forest Cat mix, is none-too-happy with the interlopers, but
he will get used to them ... and we plan to keep these cats. After what we
went through to get them out, there's no way we could give them up, the
bonding was like immediate.

I called our vet, who we have a really good relationship with, who gave us a
bit of advice and, even though it was after hours, let us know that one of
their techs would be at the office at a certain time and for us to take the
cats down there to have them checked out.

They were both alert and active ... and quite flea-bitten, they put some
Advantage on them even though they're still probably too young for Advantage
.... and seem to be none the worse for the experience. We're not quite sure
how old these cats are, probably three to four weeks, they both fit in the
palm of your hand they're so tiny, and they are a bit unsteady on their
feet, but their eyes are wide open.

The vet advised us to keep them on formula for another week or two, every
three to four hours. However, we've done that during the day and it simply
is not satisfying their hunger, we feed the amount of formula the vet
suggested and within an hour they are screaming with hunger.

So, I went to the pet store and picked up a few 3-ounce cans of kitten food,
and seeing that it was very soft consistency and just to see what would
happen, I gave them some of that. And they absolutely devoured it, they were
ravenous to the point of chewing on the styrofoam plate that I served it to
them on to try to get the crumbs. And it really seems to have satisfied
their hunger a lot more than the formula was doing. Between them, they ate
half of the 3-ounce can, and this was an hour, hour-and-a-half or so after
they had formula.

I just wanted to see if anyone here could tell me ... I don't want to call
the vet after hours again with a food question, so it would be Monday before
I could find it out from them ... should we, for a specific reason or
reasons, be continuing to give these cats formula (the vet also advised an
occasional bit of Pediolyte), or should we go ahead and make the jump to
regular food, or should we maybe do a combo of the two?