Connecting batteries in series
It is never a good idea to mix batteries.  You don't even want to mix old and new batteries of the same make/model.  The reason for this is bad things can happen as the batteries run down-- even causing some batteries to be "charged" with a reverse polarity (this is very bad).

As you get more and more batteries in series you also start wanting some sort of active monitoring system to prevent these issues (even with the all same type of batteries).  Sometimes this monitoring system can even do some active cell balancing for you.

You might be able to skip this and just "go for it", but you run the risk of reduced battery life or possibly fire/sparks/smoke.  Maybe that's acceptable, maybe not.  That's up to you to decide.