I knocked this together just to show you but yes I've used tube before on a plasma machine.
So Yes I wouldn't hesitate using it router and it would be easy enough to make and very strong.

Like Jonathan suggested I'd either machine the rail surfaces or epoxy them. I'm big fan of epoxy levelling and use it all the time has it's easy for large surfaces and plenty accurate enough for DIY machine.
Weld the plates on, stitch the tubes together then epoxy and you'll have a very stiff gantry with nice flat rail surfaces.
Again like Jonathan I wouldn't use round rail, mainly because I think they are rubbish and secondly they are rubbish.!! Profiled rails make into much better and stronger machine, accurate.