Join a clan and earn those red skull, you can easily save for a few good sets of talisman. If you can at least purchase one yellow, red and blue this month. Each are a 4 set and you can only buy one of each subtype per month. That way you can have a full set next month. Warlords are also available.

Dont concern yourself that you have a mixed set, the most important thing is a full set as soon as possible. They are definetly a good buy. Your real concern you may have is enough resources to level them up.