Start here. I would bookmark that and read through it occasionally. The crankbait section, while not completely in depth is helpful.
As far as answering your question, we need to know whether you are using a spinning or casting rod. I'm going to assume a baitcaster since you mentioned a big tangle.
For your line, make sure you are using line that your reel is rated for. This should be on the packaging that came with it. Outside these ratings, the reel will perform suboptimally, similar to putting E85 in a Ferrari. It may go, but it won't be nearly as satisfying. 6 lb is a little light for crankbaits, especially for most baitcasters. I use at least 10lb for my cranks. 12 should work.
Your issue appears to be your rod. Light crankbaits do not perform well on Medium Heavy (MH) or heavier rods. Your rod should give a rating on lure weights for it to cast. Those numbers aren't gospel, but they should give you a rough idea of what to be using. The other issue is your action. With a fast action, there is very little tip movement. Combine these factors with a light crankbait and you will have the situation you are describing.
However, you should be able to at least cast a crankbait on your setup without getting a tangle. I don't know your exact situation with your setup or line, but my guess as to why the 6lb line refused to cooperate with your reel is that the reel was not designed for that light of line. The other possibility is that the line was stiffer than what you were used to, but at this point, I am just guessing.
For casting your crankbait further, either use a crankbait that fits your rods specifications or get a new rod. Crankbait rods are usually moderate to moderate-fast action. I actually just received my Falcon Bucoo cranking rod today and asked a question about pitfalls with using cranking rods that you might find interesting.
For very light crankbaits like the wood Rapalas, I use a medium fast spinning combo with 10lb test line. It works well for the lighter lures since it can handle lures all the way down to 1/8th of an ounce.
As far as attaching weight to a crankbait, I know people have done Carolina rigs with crankbaits, but I have no idea how successful they have been with it. Personally, I would not recommend it.
Hope this helped, and remember, I am by no means an expert, just someone who has done a lot of research on this site for crankbait information in the past month or so.
Good luck and tight lines!