One of the problems with buying a department store bike is that too keep prices down they usually spec them with components that are nothing like what a modern bike will have on it. For instance you don't see too many department store bikes with 8spd or 9spd drivetrains, which is pretty much a standard for bikes from a bike shop. That is why we would need to know how many speeds your bikes are before suggesting a setup. You may have to look around on ebay or something for older components that are compatible with say a 5,6,7 speed drivetrain. The mountain bike may be the hardest because any trigger shifter/brake combo's will likely be 8 or 9 spd setups. You can find older trigger shifters that worked with 7spd drivetrains, but it may be hard. Hope this helps you out, this is precisely why it is better to buy a good bike second hand rather then buying a new department store bike.
Edit: I just reread you post and you may be able to get some barend shifters for the road bike because they will work in friction mode with almost any combination.