1985 Cannondale SR300, 1985 Cannondale ST400, Gary Littlejohn Cruiser, BMX
Are you running 28's or 25's?
If you are currently running a 700x28 I don't see the problem with running a different 700x28 in your frame, what do you mean by "built-up" tire when referring to the marathon? I run 28 conti's in my SR300 Cannondale, they are tight in the fork but after about 1000 miles I can swap the front and back and have plenty of room in the fork, until then I just go with a little less air.
I wouldn't use tire liners this small, i would recommend a different tire. I run Michelin City's, they are offered in a 700x28 (I run 32's) but I haven't had one flat with them.
Are you getting glass or debris in your tires and that is why you are considering liners?