I don't see why not. As for your ride improving.... possibly but I wouldn't expect much. It also depends on what conditions your spring packs are in. If you have them I say throw them on. If you are thinking of buying them then I say save your money to do a full suspension upgrade. Then you will definitely see a difference in ride quality and I hate to tell you but going off road is mandatory.
not sure if you're considering the KYB Gas-A-Just shock in particular but they are very stiff. i have no experience with OME mediums but i've used and installed a lot of the above KYBs. they're good shocks but stiff.
as far as running two different kinds of shocks...shouldn't be a problem unless one set is significantly stiffer or softer in which case you may experience some weird handling issues.
my 60 suspension is bone stock as well except when my OE shocks went out i replaced them with Bilsteins...great shocks and not as jarring as the KYBs.
you kind of got to match the shocks to the spring, Ive heard soft shocks for stiff springs, and hard shocks for soft springs. In other words you might have to try and see what works unless you are running the same spring in the same condition as someone else in which case they would know what works for the same setup.
You're part of the way there. Generally as you increase the spring rate you decrease the compression and increase the rebound.
Going with after market shocks is a tough thing. You never know what the tuner what going for. One general rule about types of shocks is that a mono tube has the potential for better control on longer through suspensions and twin tube are cheaper but work just fine for short through vehicles like passenger cars. That's why shocks like Bilstein mono tubes generally work well on trucks. Chevy went with them a while back.