so going by ur example...
i could do 15 cards for say mono fire rush with 8 cards being pendulums and 7 cards being ?
Yes, you could, but It would produce a lot of useless quanta ofcourse, and cause a lot of bad draws.
Try this instead: Mark of Fire, 4 fire pillar/tower/pendulum, 6 fire bolt/lance, 6 darkness pendulum, 4 drain/siphon life, 6 water pendulum, 4 ice bolt/lance
I think it will be more fun to play, but you might lose to all rush decks