A b/c rods line weight is 10-20 lbs, I should be able to use 8 lb line on this rod; right?

From what I understand about this, the line would be overly stressed and might break before the rod reached it's proper bend radius when fighting a fish.  You might never have a problem though depending on how large the fish are you are catching.  Anybody's guess, and mine isn't worth too much.

I'm using 8 lb P-line CXX, would that make a difference in strength in using an 8 lb line w/ the rod specs I mentioned?  I understand that CXX is some pretty tough line.  I use 8 lb CXX for crankbaits/topwater & jerkbaits.

Check out the diameter of your 8-pound test and compare it with the 10-pound test offerings.

If the same, you may not have a problem.

If it is not the same, you can have problems.

Remember, the rod's specifications illustrate the correct line test and lure weight for maximum performance and casting distance.

If you go outside of these paramaters you are short-changing yourself and you could damage the rod.

I am sure you have $10 to $12 to purchase the correct line test, even if it is a small spool.

If you want the correct line test to last longer, consider using the 8-pound test as a backup on your reel and then spool on about 100 yards of the 10 or 12-pound test onto your reel.

Use a uni-knot to tie the lines together.

And double check your reels to make sure their specifications balance with the rod's specs.  

Along those same lines, using braid on some rods would fall outside the suggested parameters on both ends.  For instance, 50# braid on a rod rated for 15 to 30 pound line would have a breaking strength greater than the rod is rated for and a diameter less than or about where the rod is rated  (15# min).  Is this an issue?  I know the higher breaking strength would probably be the greater concern.

P-Line CXX is approximately 40% stronger than the

stated breaking strength: 8 X 1.40 = 11.2 lb test. #12

Yo-Zuri Hybrid is 19.5 lb test and is what I recommend

for baitcasting rigs.

8-)

As long as your drag is set properly you should have no problems. Most 8lb line is not 8lb test anyway. You might need to ease up on the hookset, as most reel's drags have significantly greater start-up torque than running torque.

Ghoti has it right...  No matter what your situation, you set the drag to account for any discrepancy.  For example if using braid, set the drag to 25%-30% of the rod's lowest line rating and if using undersized mono, set it to 25%-30% the line's rated break strength.  Always err on the lesser side.  (line or rod)

Yes there can be some exceptions but this will protect your gear and work for about 98% of the scenarios you will encounter.

Ghoti has it right... No matter what your situation, you set the drag to account for any discrepancy. For example if using braid, set the drag to 25%-30% of the rod's lowest line rating and if using undersized mono, set it to 25%-30% the line's rated break strength. Always err on the lesser side. (line or rod)

.

Same....line type or brand comes secondary, and not, you don't have buy Yo-Zuri or P-line