The problem is that e7 is very limited. Could I print a barcode using an svc interface on 3700? Sure, no problem. The TM-88 uses regular old ASCII for commands. ESC/Pos is what they call it. The problem is that I can't really answer that with e7; we so rarely sell it that I have literally no idea how it does svc interfaces. I suppose it never hurts to try.
As to the last one, again, sorry, no idea. E7 doesn't use crystal reports, instead using something built in; I don't believe there is a way to include a formula in them (though, don't quote me on that, it might be there). If it isn't a really large chain of stored you could consider setting each store to a different range of check numbers? Messy, but I'm kind of flailing around in the dark when it comes to anything more than standard on e7.