Hi Shellie,
Sounds like your vet is giving you good advice. The only thing I wouldn't do is feed tubifex worms. They are on the list of things that are not good for turtles. If you've already fed them some, don't worry about it but try to feed them other things in the future. Here's a link with everything you ever wanted to know about turtle nuitrition. Read through it and then if you have specific questions come back and we'll be glad to help:
http://www.redearslider.com/index_nutrition.html
It's very good that you were able to find Reptomin, by the way. It's a well respected brand but isn't available in some parts of the world. It can be your turtle's staple with veggies - especially green leafy things - and other treats for variety.
The amount to feed your turtles depends on their size. If a turtle is less than 4" long, the rule of thumb is that, every day, you can feed him/her the amount of pellets that would fit in his/her head (head not including neck) if it were hollow. When they get to be over 4" long, the amount can be cut back to a hollow-head-full every other day. On the days your turtles have high protein foods like the frozen seafood in a cube or shrimp or tuna, you can skip the pellets altogether.
Any uneaten pellets or other rich foods left over after a meal (not usually a problem!) should be netted out and thrown away to avoid fowling the water. Or else you can feed them the rich foods in a separate container.
They can have green leafy things like red and green leaf lettuce every day. If they aren't eating it now, just keep offering and one day they'll figure out it's good. Leave the veggies in the tank throughout the day for them to graze on when the mood stricks. Carrots, berries, tomato, sweet potato and the like can be offered a couple of times a week.
When you have time we'd LOVE to see pictures of the little guys!