Chain tension

[skippy]

I ride with my chain on the middle chainring/middle cogs on the trail, but I found that when I shift to smaller cogs, my tension is really low and my chain jumps off occasionally.

So ok, my main question is:

If I don't plan on using the big cog (I know some of you don't even have them...), is it ok to take enough links out so that the top limit of my rear derailleur is middle chainring/big cog?

Secondary question:

If I use an SRAM chain with the quicklink (speedlink?), does it make sense to have two chains -- so that if I go on a road ride or something, I can change to the longer chain?
(Now, I know the easiest thing would be to buy second bike... but unless someone wants to throw several hundred dollars my way...)

Thanks!