As someone who used to ski switch on the ole straight ski with no upturned tail, I do not reccomend skiing switch without some degree of upturned tail.
Why? Cause it really really hurts when the back of the ski digs in and sends you backwards at a fast rate landing on the back of your head. Simple enough !
Can you do it? Yes you can. Still don't reccomend it.
Skiing backwards takes time to get accustomed to. During this learning process you will surely dig in the tails if they are not upturned.
Keeping a flat ski and staying balanced is one thing going straight. making paralell turns switch is something else and as you learn how to do it you will screw up and lean backwards. Again with a non upturned tail WHAMOO !
Jumping forget it unless you have a death wish. Just buy twin tips or a ski with up turned tail, save your self the hospital bills.
Go to a store that sells used equipment and buy the junkiest cheapest twin tip you can find to learn on, then decide if you really want twin tips.
There are many many many great skis out there that are not park specific with up turned tails. Hmm lets say the 8800